diff --git a/src/obikpartitionner/src/partition.rs b/src/obikpartitionner/src/partition.rs
index cf6370f..61698e2 100644
--- a/src/obikpartitionner/src/partition.rs
+++ b/src/obikpartitionner/src/partition.rs
@@ -509,7 +509,11 @@ fn build_mphf(unique_path: &Path, f0: usize) -> io::Result<Mphf> {
     let kmers: &[u64] = unsafe {
         std::slice::from_raw_parts(mmap.as_ptr() as *const u64, f0)
     };
-    Ok(Mphf::new_from_par_iter(f0, kmers.par_iter().copied(), PtrHashParams::<CubicEps>::default()))
+    // Sequential constructor: the outer par_iter over partitions already saturates
+    // the Rayon pool. new_from_par_iter would get no additional threads and adds
+    // coordination overhead. try_new accesses the same mmap'd pages at zero extra cost.
+    Mphf::try_new(kmers, PtrHashParams::<CubicEps>::default())
+        .ok_or_else(|| io::Error::other("ptr_hash construction failed"))
 }
 
 fn count_partition(dir: &Path, dedup_path: &Path, chunk_kmers: usize) -> SKResult<()> {