title: - (void)scrollViewDidScroll:(UIScrollView *)scrollView too slow in iOS 8
body: <p>I have a problem where scrolling up/down and setting the contentoffset from within <code>scrollviewdidscroll</code> on a secondary <code>scrollview</code> causes a minor jittery behaviour... or more accurately a low frame rate.</p>
  
  <p>I currently rely on <code>scrollViewDidScroll</code> to manage effects such as Parallax in my <code>UIScrollView</code>, these effects are applied by listening for <code>scrollViewDidScroll</code>, but the turn around time for each call of this method is (for some reason), too slow and causes enough of a delay for it to look kind of bad when scrolling. </p>
  
  <p>Interestingly, <code>iOS 9</code>, runs fine.</p>
  
  <p>I've tried alternative methods, such as turning off images or using <code>AsyncDisplayKit</code> but both have no effect on the number of times <code>scrollViewDidScroll</code> is fired.</p>
  
  <p>It looks to me that I may need to rearchitect the way I create the parallax effect, but I'm hesitant to in case there is a quick fix.</p>