diff --git a/crates/fidc-core/src/data.rs b/crates/fidc-core/src/data.rs
index 153e97a..27d33f0 100644
--- a/crates/fidc-core/src/data.rs
+++ b/crates/fidc-core/src/data.rs
@@ -566,7 +566,9 @@ struct SymbolPriceSeries {
     valid_volume_start_by_count: Vec<usize>,
 }
 
-type DenseRowPositionIndex = BTreeMap<NaiveDate, Vec<u32>>;
+// Row groups keep their chronological BTreeMap for range scans, while the
+// hot symbol-id lookup uses a hash index to avoid a tree traversal per row.
+type DenseRowPositionIndex = AHashMap<NaiveDate, Vec<u32>>;
 
 const MISSING_ROW_POSITION: u32 = u32::MAX;
 const MAX_DENSE_ROW_INDEX_BYTES: usize = 256 * 1024 * 1024;
@@ -4048,7 +4050,7 @@ fn build_dense_row_positions<T>(
         return None;
     }
 
-    let mut positions_by_date = BTreeMap::new();
+    let mut positions_by_date = AHashMap::new();
     for (date, rows) in groups {
         let symbol_ids = symbol_ids_by_date.get(date)?;
         if rows.len() != symbol_ids.len() {