#1089 properly stated on all API docs that the progressBlock is called on a background queue

SDWebImage/FLAnimatedImage/FLAnimatedImageView+WebCache.h

@@ -116,6 +116,7 @@
  *  @param placeholder    The image to be set initially, until the image request finishes.
  *  @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
  *  @param progressBlock  A block called while image is downloading
+ *                        @note the progress block is executed on a background queue
  *  @param completedBlock A block called when operation has been completed. This block has no return value
  *                        and takes the requested UIImage as first parameter. In case of error the image parameter
  *                        is nil and the second parameter may contain an NSError. The third parameter is a Boolean

SDWebImage/SDWebImageDownloader.h

@@ -191,6 +191,7 @@ typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterB
  * @param url            The URL to the image to download
  * @param options        The options to be used for this download
  * @param progressBlock  A block called repeatedly while the image is downloading
+ *                       @note the progress block is executed on a background queue
  * @param completedBlock A block called once the download is completed.
  *                       If the download succeeded, the image parameter is set, in case of error,
  *                       error parameter is set with the error. The last parameter is always YES

SDWebImage/SDWebImageManager.h

@@ -193,6 +193,7 @@ SDWebImageManager *manager = [SDWebImageManager sharedManager];
  * @param url            The URL to the image
  * @param options        A mask to specify options to use for this request
  * @param progressBlock  A block called while image is downloading
+ *                       @note the progress block is executed on a background queue
  * @param completedBlock A block called when operation has been completed.
  *
  *   This parameter is required.

SDWebImage/UIImageView+HighlightedWebCache.h

@@ -77,6 +77,7 @@
  * @param url            The url for the image.
  * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
  * @param progressBlock  A block called while image is downloading
+ *                       @note the progress block is executed on a background queue
  * @param completedBlock A block called when operation has been completed. This block has no return value
  *                       and takes the requested UIImage as first parameter. In case of error the image parameter
  *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean

SDWebImage/UIImageView+WebCache.h

@@ -141,6 +141,7 @@
  * @param placeholder    The image to be set initially, until the image request finishes.
  * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
  * @param progressBlock  A block called while image is downloading
+ *                       @note the progress block is executed on a background queue
  * @param completedBlock A block called when operation has been completed. This block has no return value
  *                       and takes the requested UIImage as first parameter. In case of error the image parameter
  *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
@@ -162,6 +163,7 @@
  * @param placeholder    The image to be set initially, until the image request finishes.
  * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
  * @param progressBlock  A block called while image is downloading
+ *                       @note the progress block is executed on a background queue
  * @param completedBlock A block called when operation has been completed. This block has no return value
  *                       and takes the requested UIImage as first parameter. In case of error the image parameter
  *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean

SDWebImage/UIView+WebCache.h

@@ -35,6 +35,7 @@ typedef void(^SDSetImageBlock)(UIImage * _Nullable image, NSData * _Nullable ima
  * @param operationKey   A string to be used as the operation key. If nil, will use the class name
  * @param setImageBlock  Block used for custom set image code
  * @param progressBlock  A block called while image is downloading
+ *                       @note the progress block is executed on a background queue
  * @param completedBlock A block called when operation has been completed. This block has no return value
  *                       and takes the requested UIImage as first parameter. In case of error the image parameter
  *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean