diff --git a/src/obicompactvec/src/bitmatrix/columnar.rs b/src/obicompactvec/src/bitmatrix/columnar.rs
index 273cc773..0b05fc1a 100644
--- a/src/obicompactvec/src/bitmatrix/columnar.rs
+++ b/src/obicompactvec/src/bitmatrix/columnar.rs
@@ -2,13 +2,12 @@ use std::io;
 use std::path::Path;
 
 use ndarray::{Array1, Array2};
-use rayon::prelude::*;
 
 use crate::bitvec::{PersistentBitVec, PersistentBitVecBuilder};
 use crate::meta::MatrixMeta;
 
 use super::col_path;
-use super::pairwise::{pairwise_matrix, pairwise2_matrix};
+use super::pairwise::{fill_row_generic, par_col_reduce, pairwise2_matrix, pairwise_matrix, row_generic};
 
 // ── ColumnarBitMatrix ─────────────────────────────────────────────────────────
 
@@ -36,22 +35,16 @@ impl ColumnarBitMatrix {
 
     #[inline]
     pub(crate) fn row(&self, slot: usize) -> Box<[bool]> {
-        self.cols.iter().map(|c| c.get(slot)).collect()
+        row_generic(slot, self.n_cols(), |c, s| self.col(c).get(s))
     }
 
     #[inline]
     pub(crate) fn fill_row(&self, slot: usize, buf: &mut [u32]) {
-        for (c, col) in self.cols.iter().enumerate() {
-            buf[c] = col.get(slot) as u32;
-        }
+        fill_row_generic(slot, buf, |c, s| self.col(c).get(s) as u32)
     }
 
     pub(crate) fn count_ones(&self) -> Array1<u64> {
-        let counts: Vec<u64> = (0..self.n_cols())
-            .into_par_iter()
-            .map(|c| self.col(c).count_ones())
-            .collect();
-        Array1::from_vec(counts)
+        par_col_reduce(self.n_cols(), |c| self.col(c).count_ones())
     }
 
     pub(crate) fn partial_jaccard_dist_matrix(&self) -> (Array2<u64>, Array2<u64>) {
diff --git a/src/obicompactvec/src/bitmatrix/mod.rs b/src/obicompactvec/src/bitmatrix/mod.rs
index 6435ca2b..40bb06e9 100644
--- a/src/obicompactvec/src/bitmatrix/mod.rs
+++ b/src/obicompactvec/src/bitmatrix/mod.rs
@@ -24,7 +24,7 @@ pub use packed::pack_bit_matrix;
 pub use persistent::PersistentBitMatrix;
 pub use sparse::{PersistentSparseBitMatrix, PersistentSparseBitMatrixBuilder, pack_sparse_bit_matrix};
 
-pub(crate) use pairwise::{pairwise_matrix, pairwise2_matrix};
+pub(crate) use pairwise::{fill_row_generic, par_col_reduce, pairwise2_matrix, pairwise_matrix, row_generic};
 
 fn col_path(dir: &Path, col: usize) -> PathBuf {
     dir.join(format!("col_{col:06}.pbiv"))
diff --git a/src/obicompactvec/src/bitmatrix/packed.rs b/src/obicompactvec/src/bitmatrix/packed.rs
index 840abdb5..fcae6466 100644
--- a/src/obicompactvec/src/bitmatrix/packed.rs
+++ b/src/obicompactvec/src/bitmatrix/packed.rs
@@ -4,14 +4,13 @@ use std::path::Path;
 
 use memmap2::Mmap;
 use ndarray::{Array1, Array2};
-use rayon::prelude::*;
 
 use crate::bitvec::PersistentBitVecBuilder;
 use crate::meta::MatrixMeta;
 use crate::views::BitSliceView;
 
 use super::col_path;
-use super::pairwise::{pairwise_matrix, pairwise2_matrix};
+use super::pairwise::{fill_row_generic, par_col_reduce, pairwise2_matrix, pairwise_matrix, row_generic};
 
 // ── PackedBitMatrix ───────────────────────────────────────────────────────────
 
@@ -53,16 +52,12 @@ impl PackedBitMatrix {
 
     #[inline]
     pub(crate) fn fill_row(&self, slot: usize, buf: &mut [u32]) {
-        for (c, &data_off) in self.data_offsets.iter().enumerate() {
-            buf[c] = ((self.mmap[data_off + (slot >> 3)] >> (slot & 7)) & 1) as u32;
-        }
+        fill_row_generic(slot, buf, |c, s| self.col_slice(c).get(s) as u32)
     }
 
     #[inline]
     pub(crate) fn row(&self, slot: usize) -> Box<[bool]> {
-        (0..self.n_cols).map(|c| {
-            (self.mmap[self.data_offsets[c] + (slot >> 3)] >> (slot & 7)) & 1 != 0
-        }).collect()
+        row_generic(slot, self.n_cols, |c, s| self.col_slice(c).get(s))
     }
 
     #[inline]
@@ -91,11 +86,7 @@ impl PackedBitMatrix {
     }
 
     pub(crate) fn count_ones(&self) -> Array1<u64> {
-        Array1::from_vec(
-            (0..self.n_cols).into_par_iter()
-                .map(|c| self.col_slice(c).count_ones())
-                .collect()
-        )
+        par_col_reduce(self.n_cols, |c| self.col_slice(c).count_ones())
     }
 
     pub(crate) fn partial_jaccard_dist_matrix(&self) -> (Array2<u64>, Array2<u64>) {
diff --git a/src/obicompactvec/src/bitmatrix/pairwise.rs b/src/obicompactvec/src/bitmatrix/pairwise.rs
index dba8079c..4873cd30 100644
--- a/src/obicompactvec/src/bitmatrix/pairwise.rs
+++ b/src/obicompactvec/src/bitmatrix/pairwise.rs
@@ -1,8 +1,30 @@
-use ndarray::Array2;
+use ndarray::{Array1, Array2};
 use rayon::prelude::*;
 
 // ── Shared matrix helpers (also used by intmatrix.rs) ─────────────────────────
 
+/// Shared `row`/`fill_row` bodies for matrix backends (`Columnar`/`Packed`,
+/// bit or int) whose only real difference is how to fetch one column's
+/// value at a slot — reused so the same `(0..n_cols).map(|c| get(c,
+/// slot))` loop isn't hand-written once per format, mirroring
+/// `views::nonzero_triples`'s reuse across the same backends.
+pub(crate) fn row_generic<T: Copy>(slot: usize, n_cols: usize, get: impl Fn(usize, usize) -> T) -> Box<[T]> {
+    (0..n_cols).map(|c| get(c, slot)).collect()
+}
+
+/// Like [`row_generic`], filling a caller-provided buffer instead of allocating.
+pub(crate) fn fill_row_generic<T: Copy>(slot: usize, buf: &mut [T], get: impl Fn(usize, usize) -> T) {
+    for (c, out) in buf.iter_mut().enumerate() { *out = get(c, slot); }
+}
+
+/// Shared per-column parallel reduction: `count_ones`/`sum`/`count_nonzero`
+/// all have this exact shape (`(0..n_cols).into_par_iter().map(f).collect()`)
+/// across every `Columnar`/`Packed` backend — written once here instead of
+/// once per format.
+pub(crate) fn par_col_reduce<T: Send>(n_cols: usize, f: impl Fn(usize) -> T + Sync + Send) -> Array1<T> {
+    Array1::from_vec((0..n_cols).into_par_iter().map(f).collect())
+}
+
 fn upper_pairs(n: usize) -> Vec<(usize, usize)> {
     (0..n).flat_map(|i| (i + 1..n).map(move |j| (i, j))).collect()
 }
diff --git a/src/obicompactvec/src/builder.rs b/src/obicompactvec/src/builder.rs
index 72bdc95b..e682f9b6 100644
--- a/src/obicompactvec/src/builder.rs
+++ b/src/obicompactvec/src/builder.rs
@@ -192,20 +192,33 @@ impl PersistentCompactIntVecBuilder {
     }
 
     pub fn add(&mut self, other: IntSliceView<'_>) {
-        let n = self.n;
-        for s in 0..n {
-            let sb = self.primary_bytes()[s];
-            let ob = other.primary_bytes()[s];
-            if sb < 255 && ob < 255 {
-                let sum = sb as u32 + ob as u32;
-                if sum < 255 { self.primary_bytes_mut()[s] = sum as u8; }
-                else         { self.set(s, sum); }
+        // Fast path: both operands fit in a byte — mutate in place, zipped,
+        // no repeated slice-indexing per element. Slots that already
+        // overflow (either side) or newly overflow (sum ≥ 255) are
+        // collected here and settled after the loop, via `get`/`set`
+        // (which need the overflow map, not touchable while it's borrowed
+        // through this loop).
+        let mut newly_overflow: Vec<(usize, u32)> = Vec::new();
+        let mut already_overflow: Vec<usize> = Vec::new();
+        for (s, (a, &b)) in self.primary_bytes_mut().iter_mut().zip(other.primary_bytes()).enumerate() {
+            if *a < 255 && b < 255 {
+                let sum = *a as u32 + b as u32;
+                if sum < 255 {
+                    *a = sum as u8;
+                } else {
+                    newly_overflow.push((s, sum));
+                    *a = 255;
+                }
             } else {
-                let sv = self.get(s);
-                let ov = other.get(s);
-                self.set(s, sv + ov);
+                already_overflow.push(s);
             }
         }
+        for (s, sum) in newly_overflow { self.overflow.insert(s, sum); }
+        for s in already_overflow {
+            let sv = self.get(s);
+            let ov = other.get(s);
+            self.set(s, sv + ov);
+        }
     }
 
     pub fn min(&mut self, other: IntSliceView<'_>) {
@@ -233,18 +246,26 @@ impl PersistentCompactIntVecBuilder {
     }
 
     pub fn diff(&mut self, other: IntSliceView<'_>) {
-        let n = self.n;
-        for s in 0..n {
-            let sb = self.primary_bytes()[s];
-            let ob = other.primary_bytes()[s];
-            if sb < 255 {
-                self.primary_bytes_mut()[s] = if ob < 255 { sb.saturating_sub(ob) } else { 0 };
+        // Same shape as `add`: fast byte-level path zipped in place: when
+        // `self` isn't overflow, either `other` isn't either (plain
+        // saturating subtract) or `other`'s true value is ≥ 255 — known
+        // larger than any non-overflow `self` value, so the result
+        // saturates to 0 without needing `other.get`. Only `self`-overflow
+        // slots need the overflow map, settled after the loop.
+        let mut needs_full: Vec<usize> = Vec::new();
+        for (s, (a, &b)) in self.primary_bytes_mut().iter_mut().zip(other.primary_bytes()).enumerate() {
+            if *a < 255 {
+                *a = if b < 255 { a.saturating_sub(b) } else { 0 };
             } else {
-                let sv = self.get(s);
-                let ov = if ob < 255 { ob as u32 } else { other.get(s) };
-                self.set(s, sv.saturating_sub(ov));
+                needs_full.push(s);
             }
         }
+        for s in needs_full {
+            let sv = self.get(s);
+            let ob = other.primary_bytes()[s];
+            let ov = if ob < 255 { ob as u32 } else { other.get(s) };
+            self.set(s, sv.saturating_sub(ov));
+        }
     }
 
     pub fn mask_with(&mut self, mask: BitSliceView<'_>) {
diff --git a/src/obicompactvec/src/intmatrix.rs b/src/obicompactvec/src/intmatrix.rs
index deb2c31f..dbb36338 100644
--- a/src/obicompactvec/src/intmatrix.rs
+++ b/src/obicompactvec/src/intmatrix.rs
@@ -4,9 +4,8 @@ use std::path::{Path, PathBuf};
 
 use memmap2::Mmap;
 use ndarray::{Array1, Array2};
-use rayon::prelude::*;
 
-use crate::bitmatrix::{pairwise_matrix, pairwise2_matrix};
+use crate::bitmatrix::{fill_row_generic, par_col_reduce, pairwise2_matrix, pairwise_matrix, row_generic};
 use crate::builder::PersistentCompactIntVecBuilder;
 use crate::colgroup::{chunked_presence_count, ColGroup, MatrixGroupOps};
 use crate::format::{HEADER_SIZE, OVERFLOW_ENTRY_SIZE};
@@ -45,28 +44,20 @@ impl ColumnarCompactIntMatrix {
 
     #[inline]
     pub(crate) fn row(&self, slot: usize) -> Box<[u32]> {
-        self.cols.iter().map(|c| c.get(slot)).collect()
+        row_generic(slot, self.n_cols(), |c, s| self.col(c).get(s))
     }
 
     #[inline]
     pub(crate) fn fill_row(&self, slot: usize, buf: &mut [u32]) {
-        for (c, col) in self.cols.iter().enumerate() { buf[c] = col.get(slot); }
+        fill_row_generic(slot, buf, |c, s| self.col(c).get(s))
     }
 
     pub(crate) fn sum(&self) -> Array1<u64> {
-        let sums: Vec<u64> = (0..self.n_cols())
-            .into_par_iter()
-            .map(|c| self.col(c).sum())
-            .collect();
-        Array1::from_vec(sums)
+        par_col_reduce(self.n_cols(), |c| self.col(c).sum())
     }
 
     pub(crate) fn count_nonzero(&self) -> Array1<u64> {
-        let counts: Vec<u64> = (0..self.n_cols())
-            .into_par_iter()
-            .map(|c| self.col(c).count_nonzero())
-            .collect();
-        Array1::from_vec(counts)
+        par_col_reduce(self.n_cols(), |c| self.col(c).count_nonzero())
     }
 
     pub(crate) fn partial_bray_dist_matrix(&self) -> Array2<u64> {
@@ -166,24 +157,20 @@ impl PackedCompactIntMatrix {
 
     #[inline]
     pub(crate) fn fill_row(&self, slot: usize, buf: &mut [u32]) {
-        for c in 0..self.n_cols { buf[c] = self.get(c, slot); }
+        fill_row_generic(slot, buf, |c, s| self.get(c, s))
     }
 
     #[inline]
     pub(crate) fn row(&self, slot: usize) -> Box<[u32]> {
-        (0..self.n_cols).map(|c| self.get(c, slot)).collect()
+        row_generic(slot, self.n_cols, |c, s| self.get(c, s))
     }
 
     pub(crate) fn sum(&self) -> Array1<u64> {
-        Array1::from_vec(
-            (0..self.n_cols).into_par_iter().map(|c| self.col_view(c).sum()).collect()
-        )
+        par_col_reduce(self.n_cols, |c| self.col_view(c).sum())
     }
 
     pub(crate) fn count_nonzero(&self) -> Array1<u64> {
-        Array1::from_vec(
-            (0..self.n_cols).into_par_iter().map(|c| self.col_view(c).count_nonzero()).collect()
-        )
+        par_col_reduce(self.n_cols, |c| self.col_view(c).count_nonzero())
     }
 
     // Every pair formula below delegates to `IntSliceView` (`views.rs`) —
diff --git a/src/obicompactvec/src/tests/mod.rs b/src/obicompactvec/src/tests/mod.rs
index e5cd74cd..ee22d26e 100644
--- a/src/obicompactvec/src/tests/mod.rs
+++ b/src/obicompactvec/src/tests/mod.rs
@@ -331,6 +331,41 @@ fn threshold_jaccard_dist_with_threshold() {
     assert!((d - 2.0 / 3.0).abs() < 1e-12, "got {d}");
 }
 
+/// Exercises `partial_bray_dist`/`partial_euclidean_dist`/
+/// `partial_threshold_jaccard_dist`'s bulk-over-primary-bytes-then-correct
+/// path (see `views::IntSliceView::for_each_overflow_pair`) against a naive
+/// per-slot reference computed via `get`. Overflow slots are scattered so
+/// every union pattern the merge walk must handle is covered: overflow only
+/// in `a`, only in `b`, in both at the same slot, and in both at different
+/// slots — plus plain non-overflow slots in between.
+#[test]
+fn partial_dists_bulk_path_matches_naive_reference_with_overflow() {
+    let a = [10u32, 300, 20, 1000, 5, 500, 0, 254];
+    let b = [400u32, 7, 20, 1000, 600, 500, 255, 254];
+    let (_da, ra) = make_pciv(&a);
+    let (_db, rb) = make_pciv(&b);
+
+    let naive_bray: u64 = a.iter().zip(&b).map(|(&x, &y)| x.min(y) as u64).sum();
+    assert_eq!(ra.partial_bray_dist(&rb), naive_bray);
+
+    let naive_euclidean: f64 = a.iter().zip(&b)
+        .map(|(&x, &y)| { let d = x as f64 - y as f64; d * d })
+        .sum();
+    assert!((ra.partial_euclidean_dist(&rb) - naive_euclidean).abs() < 1e-9);
+
+    for threshold in [1u32, 6, 255, 600] {
+        let (naive_inter, naive_union) = a.iter().zip(&b)
+            .fold((0u64, 0u64), |(i, u), (&x, &y)| {
+                let xp = x >= threshold;
+                let yp = y >= threshold;
+                (i + (xp & yp) as u64, u + (xp | yp) as u64)
+            });
+        let (inter, union) = ra.partial_threshold_jaccard_dist(&rb, threshold);
+        assert_eq!(inter, naive_inter, "threshold {threshold}");
+        assert_eq!(union, naive_union, "threshold {threshold}");
+    }
+}
+
 #[test]
 fn mixed_large_dataset() {
     let n    = 1000usize;
diff --git a/src/obicompactvec/src/views.rs b/src/obicompactvec/src/views.rs
index dc80dc74..93113c4e 100644
--- a/src/obicompactvec/src/views.rs
+++ b/src/obicompactvec/src/views.rs
@@ -249,10 +249,35 @@ impl<'a> IntSliceView<'a> {
     }
 
     // ── Distance methods ──────────────────────────────────────────────────────
+    //
+    // `partial_bray_dist`, `partial_euclidean_dist` and
+    // `partial_threshold_jaccard_dist` are the hot path — called
+    // `O(n_cols²)` times per distance matrix over `n`-long columns. Each
+    // computes its formula in one branch-free pass over the raw primary
+    // bytes (vectorises far better than the element-wise `iter().zip()`
+    // used elsewhere, which re-checks the overflow sentinel on every
+    // element), then patches the result for the — normally rare — slots
+    // where that was wrong: any slot that's an overflow entry in `self` or
+    // `other`. Mirrors `format::byte_sum`'s existing "bulk over primary
+    // bytes, then correct for overflow" shape, generalised from a linear
+    // reduction (sum) to arbitrary pairwise formulas via
+    // [`for_each_overflow_pair`](Self::for_each_overflow_pair).
+    // `partial_relfreq_*`/`partial_hellinger_*` are left on the
+    // element-wise path below: they already need a full pass to normalise
+    // by `sum_a`/`sum_b`, so the bulk-primary-bytes trick buys much less there.
 
     pub fn partial_bray_dist(self, other: IntSliceView<'_>) -> u64 {
         assert_eq!(self.n, other.n, "length mismatch");
-        self.iter().zip(other.iter()).map(|(a, b)| a.min(b) as u64).sum()
+        let term = |a: u32, b: u32| (a as u64).min(b as u64);
+        let mut total: u64 = self.primary.iter().zip(other.primary)
+            .map(|(&a, &b)| term(a as u32, b as u32))
+            .sum();
+        self.for_each_overflow_pair(other, |slot, a_true, b_true| {
+            let wrong = term(self.primary[slot] as u32, other.primary[slot] as u32);
+            let right = term(a_true, b_true);
+            total = total + right - wrong;
+        });
+        total
     }
 
     pub fn bray_dist(self, other: IntSliceView<'_>) -> f64 {
@@ -281,9 +306,16 @@ impl<'a> IntSliceView<'a> {
 
     pub fn partial_euclidean_dist(self, other: IntSliceView<'_>) -> f64 {
         assert_eq!(self.n, other.n, "length mismatch");
-        self.iter().zip(other.iter())
-            .map(|(a, b)| { let d = a as f64 - b as f64; d * d })
-            .sum()
+        let term = |a: u32, b: u32| { let d = a as f64 - b as f64; d * d };
+        let mut total: f64 = self.primary.iter().zip(other.primary)
+            .map(|(&a, &b)| term(a as u32, b as u32))
+            .sum();
+        self.for_each_overflow_pair(other, |slot, a_true, b_true| {
+            let wrong = term(self.primary[slot] as u32, other.primary[slot] as u32);
+            let right = term(a_true, b_true);
+            total += right - wrong;
+        });
+        total
     }
 
     pub fn euclidean_dist(self, other: IntSliceView<'_>) -> f64 {
@@ -334,12 +366,23 @@ impl<'a> IntSliceView<'a> {
 
     pub fn partial_threshold_jaccard_dist(self, other: IntSliceView<'_>, threshold: u32) -> (u64, u64) {
         assert_eq!(self.n, other.n, "length mismatch");
-        self.iter().zip(other.iter())
-            .fold((0u64, 0u64), |(inter, uni), (a, b)| {
-                let ap = a >= threshold;
-                let bp = b >= threshold;
-                (inter + (ap & bp) as u64, uni + (ap | bp) as u64)
-            })
+        let term = |a: u32, b: u32| {
+            let ap = a >= threshold;
+            let bp = b >= threshold;
+            ((ap & bp) as u64, (ap | bp) as u64)
+        };
+        let (mut inter, mut uni) = self.primary.iter().zip(other.primary)
+            .fold((0u64, 0u64), |(inter, uni), (&a, &b)| {
+                let (ti, tu) = term(a as u32, b as u32);
+                (inter + ti, uni + tu)
+            });
+        self.for_each_overflow_pair(other, |slot, a_true, b_true| {
+            let (wi, wu) = term(self.primary[slot] as u32, other.primary[slot] as u32);
+            let (ri, ru) = term(a_true, b_true);
+            inter = inter + ri - wi;
+            uni = uni + ru - wu;
+        });
+        (inter, uni)
     }
 
     pub fn threshold_jaccard_dist(self, other: IntSliceView<'_>, threshold: u32) -> f64 {
@@ -350,6 +393,38 @@ impl<'a> IntSliceView<'a> {
     pub fn jaccard_dist(self, other: IntSliceView<'_>) -> f64 {
         self.threshold_jaccard_dist(other, 1)
     }
+
+    /// Walks the union of `self`'s and `other`'s overflow slots — the
+    /// slots where a bulk pass over raw primary bytes computed the wrong
+    /// term because a `255` sentinel isn't the true value — and calls
+    /// `correct(slot, self_true, other_true)` once per slot so the caller
+    /// can undo its (wrong) sentinel-based contribution and add the true
+    /// one. Overflow entries are sorted by slot in both views (on-disk
+    /// invariant, see `format::finalize_pciv`), so their union is walked in
+    /// one linear merge — no allocation, no full second pass over `n`.
+    fn for_each_overflow_pair(self, other: IntSliceView<'_>, mut correct: impl FnMut(usize, u32, u32)) {
+        let mut a_it = self.overflow_entries().peekable();
+        let mut b_it = other.overflow_entries().peekable();
+        loop {
+            let slot = match (a_it.peek(), b_it.peek()) {
+                (None, None) => break,
+                (Some(&(s, _)), None) => s,
+                (None, Some(&(s, _))) => s,
+                (Some(&(sa, _)), Some(&(sb, _))) => sa.min(sb),
+            };
+            let a_true = if a_it.peek().map(|&(s, _)| s) == Some(slot) {
+                a_it.next().unwrap().1
+            } else {
+                self.primary[slot] as u32
+            };
+            let b_true = if b_it.peek().map(|&(s, _)| s) == Some(slot) {
+                b_it.next().unwrap().1
+            } else {
+                other.primary[slot] as u32
+            };
+            correct(slot, a_true, b_true);
+        }
+    }
 }
 
 // ── NonzeroSlotsView / nonzero_triples ──────────────────────────────────────