The UIGraphics* methods are designed to be called from the main thread only. They are probably the source of your trouble.

You can replace UIGraphicsBeginImageContext() with a call to CGBitmapContextCreate(); it s a little more involved (you need to create a color space, figure out the right sized buffer to create, and allocate it yourself). The CG* methods are fine to run from a different thread.

I m not sure how you re initializing UIImage, but if you re doing it with imageNamed: or initWithFile: then you might be able to force it to load by loading the data yourself and then calling initWithData:. The stutter is probably due to lazy file I/O, so initializing it with a data object won t give it the option of reading from a file.

I ve had the same stuttering problem, with some help I figured out the proper solution here: Non-lazy image loading in iOS

Two important things to mention:

• Don t use UIKit methods in a worker-thread. Use CoreGraphics instead.
• Even if you have a background thread for loading and decompressing images, you ll still have a little stutter if you use the wrong bitmask for your CGBitmapContext. This are the options you have to choose (it s still a bit unclear to me why):

-

CGBitmapContextCreate(imageBuffer, width, height, 8, width*4, colourSpace,
                          kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);

I ve posted a sample project here: SwapTest, it has about the same performace as Apples Photos app for loading/displaying images.

I used @jasamer s SwapTest UIImage category to force load my large UIImage (about 3000x2100 px) in a worker thread (with NSOperationQueue). This reduces the stutter time when setting the image into the UIImageView to an acceptable value (about 0.5 sec on iPad1).

Here is SwapTest UIImage category... thanks again @jasamer :)

UIImage+ImmediateLoading.h file

@interface UIImage (UIImage_ImmediateLoading)

- (UIImage*)initImmediateLoadWithContentsOfFile:(NSString*)path;
+ (UIImage*)imageImmediateLoadWithContentsOfFile:(NSString*)path;

@end

UIImage+ImmediateLoading.m file

#import "UIImage+ImmediateLoading.h"

@implementation UIImage (UIImage_ImmediateLoading)

+ (UIImage*)imageImmediateLoadWithContentsOfFile:(NSString*)path {
    return [[[UIImage alloc] initImmediateLoadWithContentsOfFile: path] autorelease];
}

- (UIImage*)initImmediateLoadWithContentsOfFile:(NSString*)path {
    UIImage *image = [[UIImage alloc] initWithContentsOfFile:path];
    CGImageRef imageRef = [image CGImage];
    CGRect rect = CGRectMake(0.f, 0.f, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef));
    CGContextRef bitmapContext = CGBitmapContextCreate(NULL,
                                                       rect.size.width,
                                                       rect.size.height,
                                                       CGImageGetBitsPerComponent(imageRef),
                                                       CGImageGetBytesPerRow(imageRef),
                                                       CGImageGetColorSpace(imageRef),
                                                       kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little
                                                       );
    //kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little are the bit flags required so that the main thread doesn t have any conversions to do.

    CGContextDrawImage(bitmapContext, rect, imageRef);
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(bitmapContext);
    UIImage* decompressedImage = [[UIImage alloc] initWithCGImage: decompressedImageRef];
    CGImageRelease(decompressedImageRef);
    CGContextRelease(bitmapContext);
    [image release];

    return decompressedImage;
}

@end

And this is how I create NSOpeationQueue and set the image on main thread...

// Loads low-res UIImage at a given index and start loading a hi-res one in background.
// After finish loading, set the hi-res image into UIImageView. Remember, we need to 
// update UI "on main thread" otherwise its result will be unpredictable.
-(void)loadPageAtIndex:(int)index {
    prevPage = index;

    //load low-res
    imageViewForZoom.image = [images objectAtIndex:index];

    //load hi-res on another thread
    [operationQueue cancelAllOperations];  
    NSInvocationOperation *operation = [NSInvocationOperation alloc];
    filePath = [imagesHD objectAtIndex:index];
    operation = [operation initWithTarget:self selector:@selector(loadHiResImage:) object:[imagesHD objectAtIndex:index]];
    [operationQueue addOperation:operation];
    [operation release];
    operation = nil;
}

// background thread
-(void)loadHiResImage:(NSString*)file {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSLog(@"loading");

    // This doesn t load the image.
    //UIImage *hiRes = [UIImage imageNamed:file];

    // Loads UIImage. There is no UI updating so it should be thread-safe.
    UIImage *hiRes = [[UIImage alloc] initImmediateLoadWithContentsOfFile:[[NSBundle mainBundle] pathForResource:file ofType: nil]];

    [imageViewForZoom performSelectorOnMainThread:@selector(setImage:) withObject:hiRes waitUntilDone:NO];

    [hiRes release];
    NSLog(@"loaded");
    [pool release];
}

I had the same problem, even though I initialized the image using data. (I guess the data is loaded lazily, too?) I’ve succeeded to force decoding using the following category:

@interface UIImage (Loading)
- (void) forceLoad;
@end

@implementation UIImage (Loading)

- (void) forceLoad
{
    const CGImageRef cgImage = [self CGImage];  

    const int width = CGImageGetWidth(cgImage);
    const int height = CGImageGetHeight(cgImage);

    const CGColorSpaceRef colorspace = CGImageGetColorSpace(cgImage);
    const CGContextRef context = CGBitmapContextCreate(
        NULL, /* Where to store the data. NULL = don’t care */
        width, height, /* width & height */
        8, width * 4, /* bits per component, bytes per row */
        colorspace, kCGImageAlphaNoneSkipFirst);

    CGContextDrawImage(context, CGRectMake(0, 0, width, height), cgImage);
    CGContextRelease(context);
}

@end