UIScrollView loading images -makes scroller be slowly - objective-c

Question

I have a scroll view with many UIViews, and these views have UIImageViews-that i load images into, while scrolling.
So every page has 4 of these views,that are preloaded to scroller at start, and i load the next page images while scrolling.
I do that in another thread , and still, scroller not entirely scrolls fast, if you look good it has some tiny flicks .
here is the function that loads them :
-(void)loadMainImageToIndex:(int)index
{
NSDictionary *dic=[mainData objectAtIndex:index];
NSString *userImageUrl=[dic objectForKey:#"url"];
NSURL *userUrl=[NSURL URLWithString:userImageUrl];
[self downloadImageWithURL:userUrl completionBlock:^(BOOL succeeded, NSData *tdata)
{
if (succeeded)
{
//load back
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^
{
UIImage *image=[UIImage imageWithData:tdata scale:0.25];
if (image)
{
UIGraphicsBeginImageContextWithOptions(image.size, NO, image.scale);
[image drawAtPoint:CGPointZero];
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}
dispatch_async(dispatch_get_main_queue(), ^
{
UIImageView *view=[mainImagesArray objectAtIndex:index];
});
}
}];
The url is loaded Asynch.

Answer 1

Turned out that the views shadow cause this thing :
//self.layer.shadowOffset=CGSizeMake(6.0, 6.0);
//self.layer.shadowColor=[UIColor blackColor].CGColor;
//self.layer.shadowRadius = 3;
//self.layer.shadowOpacity = 0.7;
Removing this solves the problem.
self.layer.shouldRasterize = YES; //solve the problem,