The time it takes to get the image from the IO read to the render to the imageView

hello,i want to caculate the time it takes get the image from io util render to UIImageView ,my code like this:



NSDate *ad = [NSDate date];
 NSData *data = [NSData dataWithContentsOfFile:downloadURL.relativePath];
 CFDataRef  dataRef = (__bridge CFDataRef)data;
 CGImageSourceRef source =CGImageSourceCreateWithData(dataRef, nil);
CGImageRef cgImage =CGImageSourceCreateImageAtIndex(source, 0, nil);
CFDataRef rawData =CGDataProviderCopyData(CGImageGetDataProvider(cgImage));
 NSDate *bd = [NSDate date];
 NSLog(@"time dis  :%f",[bd timeIntervalSinceDate:ad]);


but i find that large image always take longer than small image,i can't understand ,can you give me some advice