Performance impact fix (#1661)

Source/MQTTnet/Server/Internal/MqttClientSubscriptionsManager.cs

@@ -403,23 +403,23 @@ namespace MQTTnet.Server
             CreateSubscriptionResult createSubscriptionResult,
             SubscribeResult subscribeResult)
         {
-            for (var index = retainedMessages.Count - 1; index >= 0; index--)
-            {
-                var retainedMessage = retainedMessages[index];
-                if (retainedMessage == null)
-                {
-                    continue;
-                }
-
             if (createSubscriptionResult.Subscription.RetainHandling == MqttRetainHandling.DoNotSendOnSubscribe)
             {
                 // This is a MQTT V5+ feature.
-                    continue;
+                return;
                 }
 
             if (createSubscriptionResult.Subscription.RetainHandling == MqttRetainHandling.SendAtSubscribeIfNewSubscriptionOnly && !createSubscriptionResult.IsNewSubscription)
                 {
                     // This is a MQTT V5+ feature.
+                return;
+                }
+
+            for (var index = retainedMessages.Count - 1; index >= 0; index--)
+            {
+                var retainedMessage = retainedMessages[index];
+                if (retainedMessage == null)
+                {
                     continue;
                 }