diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index fba7e1144c28..bcbc1e33b83b 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -470,6 +470,8 @@ Bug Fixes
   `IndexWriter.forceMerge` or
   `IndexWriter.addIndexes(CodecReader...)`, or reindexing entirely.
 
+* GITHUB#13799: Disable intra-merge parallelism for all structures but kNN vectors. (Ben Trent)
+
 Build
 ---------------------
 
diff --git a/lucene/core/src/java/org/apache/lucene/index/MergeState.java b/lucene/core/src/java/org/apache/lucene/index/MergeState.java
index dcf9923feb36..838699215f0d 100644
--- a/lucene/core/src/java/org/apache/lucene/index/MergeState.java
+++ b/lucene/core/src/java/org/apache/lucene/index/MergeState.java
@@ -41,7 +41,7 @@
  *
  * @lucene.experimental
  */
-public class MergeState implements Cloneable {
+public class MergeState {
 
   /** Maps document IDs from old segments to document IDs in the new segment */
   public final DocMap[] docMaps;
@@ -302,55 +302,4 @@ public MergeState(
     this.intraMergeTaskExecutor = intraMergeTaskExecutor;
     this.needsIndexSort = needsIndexSort;
   }
-
-  @Override
-  public MergeState clone() {
-    StoredFieldsReader[] storedFieldsReaders = this.storedFieldsReaders.clone();
-    TermVectorsReader[] termVectorsReaders = this.termVectorsReaders.clone();
-    NormsProducer[] normsProducers = this.normsProducers.clone();
-    DocValuesProducer[] docValuesProducers = this.docValuesProducers.clone();
-    FieldsProducer[] fieldsProducers = this.fieldsProducers.clone();
-    PointsReader[] pointsReaders = this.pointsReaders.clone();
-    KnnVectorsReader[] knnVectorsReaders = this.knnVectorsReaders.clone();
-    for (int i = 0; i < storedFieldsReaders.length; ++i) {
-      if (storedFieldsReaders[i] != null) {
-        storedFieldsReaders[i] = storedFieldsReaders[i].getMergeInstance();
-      }
-      if (termVectorsReaders[i] != null) {
-        termVectorsReaders[i] = termVectorsReaders[i].getMergeInstance();
-      }
-      if (normsProducers[i] != null) {
-        normsProducers[i] = normsProducers[i].getMergeInstance();
-      }
-      if (docValuesProducers[i] != null) {
-        docValuesProducers[i] = docValuesProducers[i].getMergeInstance();
-      }
-      if (fieldsProducers[i] != null) {
-        fieldsProducers[i] = fieldsProducers[i].getMergeInstance();
-      }
-      if (pointsReaders[i] != null) {
-        pointsReaders[i] = pointsReaders[i].getMergeInstance();
-      }
-      if (knnVectorsReaders[i] != null) {
-        knnVectorsReaders[i] = knnVectorsReaders[i].getMergeInstance();
-      }
-    }
-    return new MergeState(
-        docMaps,
-        segmentInfo,
-        mergeFieldInfos,
-        storedFieldsReaders,
-        termVectorsReaders,
-        normsProducers,
-        docValuesProducers,
-        fieldInfos,
-        liveDocs,
-        fieldsProducers,
-        pointsReaders,
-        knnVectorsReaders,
-        maxDocs,
-        infoStream,
-        intraMergeTaskExecutor,
-        needsIndexSort);
-  }
 }
diff --git a/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java b/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java
index b9f14b4e39c3..5e336c7fef09 100644
--- a/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java
+++ b/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java
@@ -17,9 +17,7 @@
 package org.apache.lucene.index;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.List;
-import java.util.concurrent.Callable;
 import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
 import org.apache.lucene.codecs.Codec;
@@ -31,7 +29,6 @@
 import org.apache.lucene.codecs.PointsWriter;
 import org.apache.lucene.codecs.StoredFieldsWriter;
 import org.apache.lucene.codecs.TermVectorsWriter;
-import org.apache.lucene.search.TaskExecutor;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.IOContext;
 import org.apache.lucene.util.InfoStream;
@@ -102,12 +99,7 @@ boolean shouldMerge() {
   }
 
   private MergeState mergeState() {
-    MergeState mergeState = this.mergeState;
-    if (Thread.currentThread() != mergeStateCreationThread) {
-      // Most merges, e.g. small merges, run in the same thread, so save the cost of pulling a clone
-      // in that case.
-      mergeState = mergeState.clone();
-    }
+    assert Thread.currentThread() == mergeStateCreationThread;
     return mergeState;
   }
 
@@ -147,8 +139,6 @@ MergeState merge() throws IOException {
             IOContext.DEFAULT,
             segmentWriteState.segmentSuffix);
 
-    TaskExecutor taskExecutor = new TaskExecutor(mergeState.intraMergeTaskExecutor);
-    List<Callable<Void>> mergingTasks = new ArrayList<>();
     if (mergeState.mergeFieldInfos.hasNorms()) {
       mergeWithLogging(this::mergeNorms, segmentWriteState, segmentReadState, "norms", numMerged);
     }
@@ -161,12 +151,7 @@ MergeState merge() throws IOException {
     }
 
     if (mergeState.mergeFieldInfos.hasPointValues()) {
-      mergingTasks.add(
-          () -> {
-            mergeWithLogging(
-                this::mergePoints, segmentWriteState, segmentReadState, "points", numMerged);
-            return null;
-          });
+      mergeWithLogging(this::mergePoints, segmentWriteState, segmentReadState, "points", numMerged);
     }
 
     if (mergeState.mergeFieldInfos.hasVectorValues()) {
@@ -179,14 +164,9 @@ MergeState merge() throws IOException {
     }
 
     if (mergeState.mergeFieldInfos.hasTermVectors()) {
-      mergingTasks.add(
-          () -> {
-            mergeWithLogging(this::mergeTermVectors, "term vectors");
-            return null;
-          });
+      mergeWithLogging(this::mergeTermVectors, "term vectors");
     }
 
-    taskExecutor.invokeAll(mergingTasks);
     // write the merged infos
     mergeWithLogging(
         this::mergeFieldInfos, segmentWriteState, segmentReadState, "field infos", numMerged);