iOS สอบถามปัญหา Table view cell + Custom cell ปัญหาแลคเวลา scroll

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
        dispatch_async(queue, ^{
                strForImg = [[NSString alloc] initWithFormat:@"https://graph.facebook.com/%@/picture?type=square",[[list objectAtIndex:indexPath.row] objectForKey:@"id"]];

                url =[NSURL URLWithString:strForImg];
                img = [UIImage imageWithData: [NSData dataWithContentsOfURL:url]];
            dispatch_sync(dispatch_get_main_queue(), ^{
                [cell.imgProfile setImage:img];

            });
        });