diff --git a/milli/src/update/new/extract/cache.rs b/milli/src/update/new/extract/cache.rs
index e1dbc601c..f34d049f9 100644
--- a/milli/src/update/new/extract/cache.rs
+++ b/milli/src/update/new/extract/cache.rs
@@ -151,10 +151,7 @@ impl<'extractor> CboCachedSorter<'extractor> {
                 .iter_mut()
                 .enumerate()
                 .map(|(bucket, map)| {
-                    let file = tempfile::tempfile()?;
-                    let writer = create_writer(CompressionType::None, None, file);
-                    let spilled = writer_into_reader(writer)?;
-                    Ok(FrozenCache { bucket, cache: FrozenMap::new(map), spilled })
+                    Ok(FrozenCache { bucket, cache: FrozenMap::new(map), spilled: Vec::new() })
                 })
                 .collect(),
             InnerCaches::Spilling(SpillingCaches { caches, spilled_entries, .. }) => caches
@@ -162,10 +159,13 @@ impl<'extractor> CboCachedSorter<'extractor> {
                 .zip(mem::take(spilled_entries))
                 .enumerate()
                 .map(|(bucket, (map, sorter))| {
-                    let file = tempfile::tempfile()?;
-                    let mut writer = create_writer(CompressionType::None, None, file);
-                    sorter.write_into_stream_writer(&mut writer)?;
-                    let spilled = writer_into_reader(writer)?;
+                    let spilled = sorter
+                        .into_reader_cursors()?
+                        .into_iter()
+                        .map(ReaderCursor::into_inner)
+                        .map(BufReader::new)
+                        .map(|bufreader| grenad::Reader::new(bufreader).map_err(Into::into))
+                        .collect::<Result<_>>()?;
                     Ok(FrozenCache { bucket, cache: FrozenMap::new(map), spilled })
                 })
                 .collect(),
@@ -275,7 +275,7 @@ impl<'extractor> SpillingCaches<'extractor> {
             }
             RawEntryMut::Vacant(_entry) => {
                 let deladd = DelAddRoaringBitmap::new_del_u32(n);
-                spill_entry_to_disk(
+                spill_entry_to_sorter(
                     &mut self.spilled_entries[bucket],
                     &mut self.deladd_buffer,
                     &mut self.cbo_buffer,
@@ -302,7 +302,7 @@ impl<'extractor> SpillingCaches<'extractor> {
             }
             RawEntryMut::Vacant(_entry) => {
                 let deladd = DelAddRoaringBitmap::new_add_u32(n);
-                spill_entry_to_disk(
+                spill_entry_to_sorter(
                     &mut self.spilled_entries[bucket],
                     &mut self.deladd_buffer,
                     &mut self.cbo_buffer,
@@ -327,7 +327,7 @@ fn compute_bucket_from_hash(buckets: usize, hash: u64) -> usize {
     hash as usize % buckets
 }
 
-fn spill_entry_to_disk(
+fn spill_entry_to_sorter(
     spilled_entries: &mut grenad::Sorter<MergeDeladdCboRoaringBitmaps>,
     deladd_buffer: &mut Vec<u8>,
     cbo_buffer: &mut Vec<u8>,
@@ -367,7 +367,24 @@ fn spill_entry_to_disk(
 pub struct FrozenCache<'a, 'extractor> {
     bucket: usize,
     cache: FrozenMap<'a, 'extractor, &'extractor [u8], DelAddRoaringBitmap, DefaultHashBuilder>,
-    spilled: grenad::Reader<BufReader<File>>,
+    spilled: Vec<grenad::Reader<BufReader<File>>>,
+}
+
+pub fn transpose_and_freeze_caches<'a, 'extractor>(
+    caches: &'a mut [Vec<CboCachedSorter<'extractor>>],
+) -> Result<Vec<Vec<FrozenCache<'a, 'extractor>>>> {
+    let width = caches.get(0).map(Vec::len).unwrap_or(0);
+    let mut bucket_caches: Vec<_> = iter::repeat_with(Vec::new).take(width).collect();
+
+    for thread_caches in caches {
+        for cache in thread_caches.iter_mut() {
+            for frozen in cache.freeze()? {
+                bucket_caches[frozen.bucket].push(frozen);
+            }
+        }
+    }
+
+    Ok(bucket_caches)
 }
 
 /// Merges the caches that must be all associated to the same bucket.
@@ -379,19 +396,19 @@ pub fn merge_caches<F>(frozen: Vec<FrozenCache>, mut iter: F) -> Result<()>
 where
     F: for<'a> FnMut(&'a [u8], DelAddRoaringBitmap) -> Result<()>,
 {
+    let mut maps = Vec::new();
+    let mut readers = Vec::new();
     let mut current_bucket = None;
-    let (mut maps, spilled): (Vec<_>, Vec<_>) = frozen
-        .into_iter()
-        .map(|FrozenCache { bucket, cache, spilled }| {
-            assert_eq!(*current_bucket.get_or_insert(bucket), bucket);
-            (cache, spilled)
-        })
-        .collect();
+    for FrozenCache { bucket, cache, ref mut spilled } in frozen {
+        assert_eq!(*current_bucket.get_or_insert(bucket), bucket);
+        maps.push(cache);
+        readers.append(spilled);
+    }
 
     // First manage the spilled entries by looking into the HashMaps,
     // merge them and mark them as dummy.
     let mut heap = BinaryHeap::new();
-    for (source_index, source) in spilled.into_iter().enumerate() {
+    for (source_index, source) in readers.into_iter().enumerate() {
         let mut cursor = source.into_cursor()?;
         if cursor.move_on_next()?.is_some() {
             heap.push(Entry { cursor, source_index });