Decode images in background thread?

I have a background thread that loads images and displays them in the main thread. I noticed that the background thread has almost nothing to do, because the actual image decoding seems to be done in the main thread:

So far I’ve tried calling [UIImage imageNamed:], [UIImage imageWithData:] and CGImageCreateWithJPEGDataProvider in the background thread with no difference. Is there a way to force the decoding to be done on the background thread?

There’s already a similar question here, but it does not help. As I wrote there, I tried the following trick:

@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);

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

@end

That works (forces the image to decode), but it also triggers an apparently expensive call to ImageIO_BGR_A_TO_RGB_A_8Bit.