diff --git a/src/java/org/apache/cassandra/index/sai/disk/v1/SegmentBuilder.java b/src/java/org/apache/cassandra/index/sai/disk/v1/SegmentBuilder.java
index 3ea1bb4642df..b354307678e9 100644
--- a/src/java/org/apache/cassandra/index/sai/disk/v1/SegmentBuilder.java
+++ b/src/java/org/apache/cassandra/index/sai/disk/v1/SegmentBuilder.java
@@ -36,6 +36,8 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import com.codahale.metrics.Histogram;
+import com.codahale.metrics.UniformReservoir;
 import io.github.jbellis.jvector.quantization.VectorCompressor;
 import org.apache.cassandra.concurrent.DebuggableThreadPoolExecutor;
 import org.apache.cassandra.concurrent.NamedThreadFactory;
@@ -78,8 +80,17 @@ public abstract class SegmentBuilder
 {
     private static final Logger logger = LoggerFactory.getLogger(SegmentBuilder.class);
 
-    /** for parallelism within a single compaction */
-    public static final ExecutorService compactionExecutor = new DebuggableThreadPoolExecutor(Runtime.getRuntime().availableProcessors(),
+    private static final int COMPACTION_EXECUTOR_NUM_THREADS = Integer.getInteger("cassandra.sai.compaction.executor.threads",
+                                                                                  Runtime.getRuntime().availableProcessors() / 2);
+
+    static {
+        logger.debug("SAI compaction executor threads: {}", COMPACTION_EXECUTOR_NUM_THREADS);
+    }
+
+    /** for parallelism within a single compaction
+     *  see comments to JVector PhysicalCoreExecutor -- HT tends to cause contention for the SIMD units
+     */
+    public static final ExecutorService compactionExecutor = new DebuggableThreadPoolExecutor(COMPACTION_EXECUTOR_NUM_THREADS,
                                                                                               1,
                                                                                               TimeUnit.MINUTES,
                                                                                               new ArrayBlockingQueue<>(10 * Runtime.getRuntime().availableProcessors()),
@@ -246,6 +257,12 @@ public boolean requiresFlush()
     public static class VectorOffHeapSegmentBuilder extends SegmentBuilder
     {
         private final CompactionGraph graphIndex;
+        // Only using the no-sample reservoir to observe the behavior without sampling. Not for production use.
+        private final Histogram totalAddTimeHist = new Histogram(new UniformReservoir(10_000_000));
+        private final Histogram addGraphTimeHist = new Histogram(new UniformReservoir(10_000_000));
+        private LongAdder totalDuration = new LongAdder();
+        private LongAdder addGraphTimeDuration = new LongAdder();
+        private LongAdder totalAdds = new LongAdder();
 
         public VectorOffHeapSegmentBuilder(IndexComponents.ForWrite components,
                                            long rowIdOffset,
@@ -284,6 +301,7 @@ protected long addInternal(List<ByteBuffer> terms, int segmentRowId)
         protected long addInternalAsync(List<ByteBuffer> terms, int segmentRowId)
         {
             assert terms.size() == 1;
+            final long start = System.nanoTime();
 
             // CompactionGraph splits adding a node into two parts:
             // (1) maybeAddVector, which must be done serially because it writes to disk incrementally
@@ -302,6 +320,7 @@ protected long addInternalAsync(List<ByteBuffer> terms, int segmentRowId)
 
             updatesInFlight.incrementAndGet();
             compactionExecutor.submit(() -> {
+                long beforeAddGraphNode = System.nanoTime();
                 try
                 {
                     long bytesAdded = result.bytesUsed + graphIndex.addGraphNode(result);
@@ -315,6 +334,14 @@ protected long addInternalAsync(List<ByteBuffer> terms, int segmentRowId)
                 finally
                 {
                     updatesInFlight.decrementAndGet();
+                    long now = System.nanoTime();
+                    totalAdds.increment();
+                    long addInternalAsyncDuration = now - start;
+                    totalDuration.add(addInternalAsyncDuration);
+                    totalAddTimeHist.update(addInternalAsyncDuration / 1000);
+                    long addGraphNodeDuration = now - beforeAddGraphNode;
+                    addGraphTimeDuration.add(addGraphNodeDuration);
+                    addGraphTimeHist.update(addGraphNodeDuration / 1000);
                 }
             });
             // bytes allocated will be approximated immediately as the average of recently added terms,
@@ -333,7 +360,21 @@ protected void flushInternal(SegmentMetadataBuilder metadataBuilder) throws IOEx
         {
             if (graphIndex.isEmpty())
                 return;
+            // Print diagnostic info
+            if (totalAdds.sum() > 0)
+            {
+                logger.info("Add internal async: average time: {} us, total time: {} us, total adds: {}", (double) totalDuration.sum() / totalAdds.sum() / 1000, totalDuration.sum() / 1000, totalAdds.sum());
+                var totalAddTimeHistSnapshot = totalAddTimeHist.getSnapshot();
+                logger.info("Add internal async time histogram: min: {} us, max: {} us, mean: {} us, median: {} us, 75th percentile: {} us, 95th percentile: {} us, 99th percentile: {} us, 99.9th percentile: {} us",
+                            totalAddTimeHistSnapshot.getMin(), totalAddTimeHistSnapshot.getMax(), totalAddTimeHistSnapshot.getMean(), totalAddTimeHistSnapshot.getMedian(), totalAddTimeHistSnapshot.get75thPercentile(), totalAddTimeHistSnapshot.get95thPercentile(), totalAddTimeHistSnapshot.get99thPercentile(), totalAddTimeHistSnapshot.get999thPercentile());
+                logger.info("Add graph node: average time {} us, total time: {} us, total adds: {}", (double) addGraphTimeDuration.sum() / totalAdds.sum() / 1000, addGraphTimeDuration.sum() / 1000, totalAdds.sum());
+                var addGraphTimeHistSnapshot = addGraphTimeHist.getSnapshot();
+                logger.info("Add graph node time histogram: min: {} us, max: {} us, mean: {} us, median: {} us, 75th percentile: {} us, 95th percentile: {} us, 99th percentile: {} us, 99.9th percentile: {} us",
+                            addGraphTimeHistSnapshot.getMin(), addGraphTimeHistSnapshot.getMax(), addGraphTimeHistSnapshot.getMean(), addGraphTimeHistSnapshot.getMedian(), addGraphTimeHistSnapshot.get75thPercentile(), addGraphTimeHistSnapshot.get95thPercentile(), addGraphTimeHistSnapshot.get99thPercentile(), addGraphTimeHistSnapshot.get999thPercentile());
+            }
+            long start = System.nanoTime();
             var componentsMetadata = graphIndex.flush();
+            logger.info("Flushing graph took {}ms", (System.nanoTime() - start) / 1_000_000);
             metadataBuilder.setComponentsMetadata(componentsMetadata);
         }