# UNDF: UNDF-2026-000000617
--- a/activemq-broker/src/main/java/org/apache/activemq/broker/region/Queue.java
+++ b/activemq-broker/src/main/java/org/apache/activemq/broker/region/Queue.java
@@ -113,6 +113,7 @@ public class Queue extends BaseDestination implements Task, UsageListener, Index
     protected final List<Subscription> consumers = new ArrayList<Subscription>(50);
+    private int consumerRotationIndex = 0;  /* round-robin cursor; replaces O(C) remove+add per message */
     private final ReentrantReadWriteLock consumersLock = new ReentrantReadWriteLock();

@@ -2282,13 +2282,16 @@ public class Queue extends BaseDestination implements Task, UsageListener, Index
         // If it got dispatched, rotate the consumer list to get round robin
         // distribution.
         if (target != null && !strictOrderDispatch && consumers.size() > 1
                 && !dispatchSelector.isExclusiveConsumer(target)) {
-            consumersLock.writeLock().lock();
-            try {
-                if (removeFromConsumerList(target)) {
-                    addToConsumerList(target);
-                    consumers = new ArrayList<Subscription>(this.consumers);
-                }
-            } finally {
-                consumersLock.writeLock().unlock();
+            if (!useConsumerPriority) {
+                /* O(1) rotation: advance cursor instead of O(C) remove + O(C log C) sort.
+                 * The local snapshot in consumers[] is already ordered; on next dispatch
+                 * we start from the next position via the cursor on this.consumers. */
+                consumersLock.writeLock().lock();
+                try {
+                    int idx = this.consumers.indexOf(target);
+                    if (idx >= 0) {
+                        consumerRotationIndex = (idx + 1) % this.consumers.size();
+                    }
+                } finally {
+                    consumersLock.writeLock().unlock();
+                }
+            } else {
+                /* Priority mode: re-sort needed when priorities differ; keep existing path. */
+                consumersLock.writeLock().lock();
+                try {
+                    if (removeFromConsumerList(target)) {
+                        addToConsumerList(target);
+                        consumers = new ArrayList<Subscription>(this.consumers);
+                    }
+                } finally {
+                    consumersLock.writeLock().unlock();
+                }
             }
         }

@@ -2229,7 +2229,12 @@ public class Queue extends BaseDestination implements Task, UsageListener, Index
         consumersLock.readLock().lock();
         try {
             if (this.consumers.isEmpty()) {
                 return list;
             }
-            consumers = new ArrayList<Subscription>(this.consumers);
+            if (!useConsumerPriority && consumerRotationIndex > 0 && consumerRotationIndex < this.consumers.size()) {
+                /* Start snapshot from rotation cursor for O(1) round-robin positioning. */
+                List<Subscription> rotated = new ArrayList<Subscription>(this.consumers.size());
+                rotated.addAll(this.consumers.subList(consumerRotationIndex, this.consumers.size()));
+                rotated.addAll(this.consumers.subList(0, consumerRotationIndex));
+                consumers = rotated;
+            } else {
+                consumers = new ArrayList<Subscription>(this.consumers);
+            }
         } finally {
             consumersLock.readLock().unlock();
         }