diff --git a/Source/MQTTnet/Server/MqttClientSessionsManager.cs b/Source/MQTTnet/Server/MqttClientSessionsManager.cs
index 8188da82..a41be4f1 100644
--- a/Source/MQTTnet/Server/MqttClientSessionsManager.cs
+++ b/Source/MQTTnet/Server/MqttClientSessionsManager.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
@@ -12,10 +13,14 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public class MqttClientSessionsManager
+    public class MqttClientSessionsManager : IDisposable
     {
         private readonly BlockingCollection<MqttEnqueuedApplicationMessage> _messageQueue = new BlockingCollection<MqttEnqueuedApplicationMessage>();
-        private readonly ConcurrentDictionary<string, MqttClientSession> _sessions = new ConcurrentDictionary<string, MqttClientSession>();
+
+        /// <summary>
+        /// manual locking dictionaries is faster than using concurrent dictionary
+        /// </summary>
+        private readonly Dictionary<string, MqttClientSession> _sessions = new Dictionary<string, MqttClientSession>();
 
         private readonly CancellationToken _cancellationToken;
 
@@ -44,12 +49,15 @@ namespace MQTTnet.Server
 
         public void Stop()
         {
-            foreach (var session in _sessions)
+            lock (_sessions)
             {
-                session.Value.Stop(MqttClientDisconnectType.NotClean);
-            }
+                foreach (var session in _sessions)
+                {
+                    session.Value.Stop(MqttClientDisconnectType.NotClean);
+                }
 
-            _sessions.Clear();
+                _sessions.Clear();
+            }
         }
 
         public Task StartSession(IMqttChannelAdapter clientAdapter)
@@ -60,10 +68,11 @@ namespace MQTTnet.Server
         public Task<IList<IMqttClientSessionStatus>> GetClientStatusAsync()
         {
             var result = new List<IMqttClientSessionStatus>();
-            foreach (var session in _sessions)
+
+            foreach (var session in GetSessions())
             {
-                var status = new MqttClientSessionStatus(this, session.Value);
-                session.Value.FillStatus(status);
+                var status = new MqttClientSessionStatus(this, session);
+                session.FillStatus(status);
 
                 result.Add(status);
             }
@@ -83,12 +92,15 @@ namespace MQTTnet.Server
             if (clientId == null) throw new ArgumentNullException(nameof(clientId));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            if (!_sessions.TryGetValue(clientId, out var session))
+            lock (_sessions)
             {
-                throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
-            }
+                if (!_sessions.TryGetValue(clientId, out var session))
+                {
+                    throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
+                }
 
-            return session.SubscribeAsync(topicFilters);
+                return session.SubscribeAsync(topicFilters);
+            }
         }
 
         public Task UnsubscribeAsync(string clientId, IList<string> topicFilters)
@@ -96,20 +108,31 @@ namespace MQTTnet.Server
             if (clientId == null) throw new ArgumentNullException(nameof(clientId));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            if (!_sessions.TryGetValue(clientId, out var session))
+            lock (_sessions)
             {
-                throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
-            }
+                if (!_sessions.TryGetValue(clientId, out var session))
+                {
+                    throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
+                }
 
-            return session.UnsubscribeAsync(topicFilters);
+                return session.UnsubscribeAsync(topicFilters);
+            }
         }
 
         public void DeleteSession(string clientId)
         {
-            _sessions.TryRemove(clientId, out _);
+            lock (_sessions)
+            {
+                _sessions.Remove(clientId);
+            }
             _logger.Verbose("Session for client '{0}' deleted.", clientId);
         }
 
+        public void Dispose()
+        {
+            _messageQueue?.Dispose();
+        }
+
         private void ProcessQueuedApplicationMessages(CancellationToken cancellationToken)
         {
             while (!cancellationToken.IsCancellationRequested)
@@ -143,7 +166,7 @@ namespace MQTTnet.Server
                         _retainedMessagesManager.HandleMessageAsync(sender?.ClientId, applicationMessage).GetAwaiter().GetResult();
                     }
 
-                    foreach (var clientSession in _sessions.Values)
+                    foreach (var clientSession in GetSessions())
                     {
                         clientSession.EnqueueApplicationMessage(enqueuedApplicationMessage.Sender, applicationMessage.ToPublishPacket());
                     }
@@ -158,6 +181,14 @@ namespace MQTTnet.Server
             }
         }
 
+        private List<MqttClientSession> GetSessions()
+        {
+            lock (_sessions)
+            {
+                return _sessions.Values.ToList();
+            }
+        }
+
         private async Task RunSession(IMqttChannelAdapter clientAdapter, CancellationToken cancellationToken)
         {
             var clientId = string.Empty;
@@ -263,7 +294,7 @@ namespace MQTTnet.Server
                 {
                     if (connectPacket.CleanSession)
                     {
-                        _sessions.TryRemove(connectPacket.ClientId, out _);
+                        _sessions.Remove(connectPacket.ClientId);
 
                         clientSession.Stop(MqttClientDisconnectType.Clean);
                         clientSession.Dispose();
diff --git a/Source/MQTTnet/Server/MqttServer.cs b/Source/MQTTnet/Server/MqttServer.cs
index 69850d59..dd732f1b 100644
--- a/Source/MQTTnet/Server/MqttServer.cs
+++ b/Source/MQTTnet/Server/MqttServer.cs
@@ -123,6 +123,8 @@ namespace MQTTnet.Server
                 _cancellationTokenSource = null;
 
                 _retainedMessagesManager = null;
+
+                _clientSessionsManager?.Dispose();
                 _clientSessionsManager = null;
             }
         }