[AS*ImageNode] dispatch to the background before scheduling image download request (perf).

AsyncDisplayKit/ASMultiplexImageNode.mm

@@ -21,6 +21,7 @@
 #import "ASLog.h"
 #import "ASPhotosFrameworkImageRequest.h"
 #import "ASEqualityHelpers.h"
+#import "ASInternalHelpers.h"
 
 #if !AS_IOS8_SDK_OR_LATER
 #error ASMultiplexImageNode can be used on iOS 7, but must be linked against the iOS 8 SDK.
@@ -751,6 +752,7 @@ typedef void(^ASMultiplexImageLoadCompletionBlock)(UIImage *image, id imageIdent
   }
 
   // Download!
+  ASPerformBlockOnBackgroundThread(^{
     if (_downloaderSupportsNewProtocol) {
       [self _setDownloadIdentifier:[_downloader downloadImageWithURL:imageURL
                                                        callbackQueue:dispatch_get_main_queue()
@@ -791,6 +793,7 @@ typedef void(^ASMultiplexImageLoadCompletionBlock)(UIImage *image, id imageIdent
                                                               [strongSelf->_delegate multiplexImageNode:weakSelf didFinishDownloadingImageWithIdentifier:imageIdentifier error:error];
                                                           }]];
     }
+  });
 }
 
 #pragma mark -

AsyncDisplayKit/ASNetworkImageNode.mm

@@ -13,6 +13,7 @@
 #import "ASDisplayNode+FrameworkPrivate.h"
 #import "ASEqualityHelpers.h"
 #import "ASThread.h"
+#import "ASInternalHelpers.h"
 
 #if PIN_REMOTE_IMAGE
 #import "ASPINRemoteImageDownloader.h"
@@ -263,6 +264,8 @@
 
 - (void)_downloadImageWithCompletion:(void (^)(UIImage *image, NSError*, id downloadIdentifier))finished
 {
+  ASPerformBlockOnBackgroundThread(^{
+    ASDN::MutexLocker l(_lock);
     if (_downloaderSupportsNewProtocol) {
       _downloadIdentifier = [_downloader downloadImageWithURL:_URL
                                                 callbackQueue:dispatch_get_main_queue()
@@ -282,6 +285,7 @@
                                                      }
                                                    }];
     }
+  });
 }
 
 - (void)_lazilyLoadImageIfNecessary