UIImageView Category, which offloads image decompression onto a background queue, then after decompression sets the image on the main queue.

UIImageView+AsyncImageSet.h

//
//  UIImageView+AsyncImageSet.h
//
//  Created by Adam Roberts on 27/11/2014.
//  Copyright (c) 2014 Enigmatic Flare Ltd. All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

#import <UIKit/UIKit.h>

@interface UIImageView (AsyncImageSet)

-(void)setCGImageAsync:(CGImageRef)imageRef;
-(void)setImageAsync:(UIImage*)image;
@end

UIImageView+AsyncImageSet.m

//
// UIImageView+AsyncImageSet.m
//
// Created by Adam Roberts on 27/11/2014.
// Copyright (c) 2014 Enigmatic Flare Ltd. All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.


#import "UIImageView+AsyncImageSet.h"

@implementation UIImageView (AsyncImageSet)

-(void)setImageAsync:(UIImage*)image{
    CGImageRef imageRef = image.CGImage;
    [self setCGImageAsync:imageRef];
}

-(void)setCGImageAsync:(CGImageRef)imageRef{
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        
        CGContextRef context = CGBitmapContextCreate(NULL,
                                                     CGImageGetWidth(imageRef),
                                                     CGImageGetHeight(imageRef),
                                                     8,
                                                     CGImageGetWidth(imageRef) * 4,
                                                     colorSpace,
                                                     kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
        CGColorSpaceRelease(colorSpace);
        
        if (!context)
            return;
    
        CGRect rect = (CGRect){CGPointZero, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef)};
        CGContextDrawImage(context, rect, imageRef);
        CGImageRef decompressedImageRef = CGBitmapContextCreateImage(context);
        CGContextRelease(context);
        
        UIImage *decompressedImage = [[UIImage alloc] initWithCGImage:decompressedImageRef];
        CGImageRelease(decompressedImageRef);
        dispatch_async(dispatch_get_main_queue(), ^{
            [self setImage:decompressedImage];
        });
 });
    
}

@end