Skip heavy TreeSet opts for the first group in FirstPassGroupingCollector

lucene/CHANGES.txt

@@ -136,6 +136,8 @@ Optimizations
 * GITHUB#15151: Use `SimScorer#score` bulk API to compute impact scores per
   block of postings. (Adrien Grand)
 
+* GITHUB#15128: Skip heavy TreeSet opts for the first group in FirstPassGroupingCollector (Binlong Gao)
+
 Bug Fixes
 ---------------------
 * GITHUB#14161: PointInSetQuery's constructor now throws IllegalArgumentException

lucene/grouping/src/java/org/apache/lucene/search/grouping/FirstPassGroupingCollector.java

@@ -288,14 +288,19 @@ public void collect(int doc) throws IOException {
       }
     }
 
-    // Remove before updating the group since lookup is done via comparators
-    // TODO: optimize this
-
     final CollectedSearchGroup<T> prevLast;
+    boolean skipHeavyOps = false;
+
     if (orderedGroups != null) {
       prevLast = orderedGroups.last();
-      orderedGroups.remove(group);
-      assert orderedGroups.size() == topNGroups - 1;
+
+      // Skip remove/add for first group
+      if (group == orderedGroups.first()) {
+        skipHeavyOps = true;
+      } else {
+        orderedGroups.remove(group);
+        assert orderedGroups.size() == topNGroups - 1;
+      }
     } else {
       prevLast = null;
     }
@@ -307,9 +312,11 @@ public void collect(int doc) throws IOException {
     spareSlot = group.comparatorSlot;
     group.comparatorSlot = tmp;
 
-    // Re-add the changed group
+    // Re-add only if we removed it
     if (orderedGroups != null) {
-      orderedGroups.add(group);
+      if (!skipHeavyOps) {
+        orderedGroups.add(group);
+      }
       assert orderedGroups.size() == topNGroups;
       final CollectedSearchGroup<?> newLast = orderedGroups.last();
       // If we changed the value of the last group, or changed which group was last, then update