Added IHandleAsync, these handlers will be run async and in parallel to each other.

NzbDrone.Common/Eventing/EventAggregator.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading.Tasks;
 using NLog;
 
 namespace NzbDrone.Common.Eventing
@@ -20,10 +21,24 @@ namespace NzbDrone.Common.Eventing
         {
             _logger.Trace("Publishing {0}", message.GetType().Name);
 
+            //call synchronous handlers first.
             foreach (var handler in _handlers().OfType<IHandle<TEvent>>())
             {
-                _logger.Trace("{0} => {1}", message.GetType().Name, handler.GetType().Name);
+                _logger.Debug("{0} -> {1}", message.GetType().Name, handler.GetType().Name);
                 handler.Handle(message);
+                _logger.Debug("{0} -#> {1}", message.GetType().Name, handler.GetType().Name);
+            }
+
+
+            foreach (var handler in _handlers().OfType<IHandleAsync<TEvent>>())
+            {
+                var handlerLocal = handler;
+                Task.Factory.StartNew(() =>
+                    {
+                        _logger.Debug("{0} ~> {1}", message.GetType().Name, handlerLocal.GetType().Name);
+                        handlerLocal.HandleAsync(message);
+                        _logger.Debug("{0} ~#> {1}", message.GetType().Name, handlerLocal.GetType().Name);
+                    });
             }
         }
     }

NzbDrone.Common/Eventing/IHandle.cs

@@ -9,14 +9,32 @@ namespace NzbDrone.Common.Eventing
     public interface IHandle<TEvent> : IHandle where TEvent : IEvent
     {
         /// <summary>
-        ///   Handles the message.
+        ///   Handles the message synchronously.
         /// </summary>
         /// <param name = "message">The message.</param>
         void Handle(TEvent message);
     }
 
+    /// <summary>
+    ///   Denotes a class which can handle a particular type of message.
+    /// </summary>
+    /// <typeparam name = "TEvent">The type of message to handle.</typeparam>
+    public interface IHandleAsync<TEvent> : IHandleAsync where TEvent : IEvent
+    {
+        /// <summary>
+        ///   Handles the message asynchronously.
+        /// </summary>
+        /// <param name = "message">The message.</param>
+        void HandleAsync(TEvent message);
+    }
+
     /// <summary>
     ///   A marker interface for classes that subscribe to messages.
     /// </summary>
     public interface IHandle { }
+
+    /// <summary>
+    ///   A marker interface for classes that subscribe to messages.
+    /// </summary>
+    public interface IHandleAsync : IHandle { }
 }

NzbDrone.Core/Jobs/Implementations/RefreshEpisodeMetadata.cs

@@ -63,7 +63,7 @@ namespace NzbDrone.Core.Jobs.Implementations
                 return;
             }
 
-            notification.CurrentMessage = String.Format("Epsiode metadata refresh completed for {0}", series.Title);
+            notification.CurrentMessage = String.Format("Episode metadata refresh completed for {0}", series.Title);
         }
     }
 }

NzbDrone.Core/MediaCover/MediaCoverService.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.Linq;
+using System.Threading;
 using NLog;
 using NzbDrone.Common;
 using NzbDrone.Common.Eventing;
@@ -9,7 +10,7 @@ using NzbDrone.Core.Tv.Events;
 
 namespace NzbDrone.Core.MediaCover
 {
-    public class MediaCoverService : IHandle<SeriesUpdatedEvent>
+    public class MediaCoverService : IHandleAsync<SeriesUpdatedEvent>
     {
         private readonly HttpProvider _httpProvider;
         private readonly DiskProvider _diskProvider;
@@ -28,7 +29,7 @@ namespace NzbDrone.Core.MediaCover
             _coverRootFolder = environmentProvider.GetMediaCoverPath();
         }
 
-        public void Handle(SeriesUpdatedEvent message)
+        public void HandleAsync(SeriesUpdatedEvent message)
         {
             EnsureCovers(message.Series);
         }