# UNDF: UNDF-2026-000000102
--- a/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/DefaultStoreFileManager.java
+++ b/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/DefaultStoreFileManager.java
@@ -17,6 +17,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
+import java.util.HashSet;
+import java.util.Set;
 import java.util.stream.Collectors;

 // ... (other imports unchanged)

@@ -229,9 +230,11 @@ public class DefaultStoreFileManager implements StoreFileManager {
   @Override
   public Collection<HStoreFile> getUnneededFiles(long maxTs, List<HStoreFile> filesCompacting) {
     ImmutableList<HStoreFile> files = storeFiles.all;
+    // Build a HashSet once for O(1) membership test inside the stream.
+    // Without this, filesCompacting.contains() is O(C) per file — O(F*C) total.
+    Set<HStoreFile> compactingSet = new HashSet<>(filesCompacting);
     // 1) We can never get rid of the last file which has the maximum seqid.
     // 2) Files that are not the latest can't become one due to (1), so the rest are fair game.
     return files.stream().limit(Math.max(0, files.size() - 1)).filter(sf -> {
       long fileTs = sf.getReader().getMaxTimestamp();
-      if (fileTs < maxTs && !filesCompacting.contains(sf)) {
+      if (fileTs < maxTs && !compactingSet.contains(sf)) {
         LOG.info("Found an expired store file {} whose maxTimestamp is {}, which is below {}",
           sf.getPath(), fileTs, maxTs);
         return true;