title: I Have bad performance on using shadow effect
body: <p>I put some image views on scroll view.
  And when I drag this scroll view, I didn't have any problems.</p>
  
  <p>But after I applied shadow effect to these image views, dragging this scroll view has bad performance.</p>
  
  <p>I used shadowOpacity, shadowRadius and shadowOffset methods.</p>
  
  <p>ex:</p>
  
  <pre><code>[ [ anImageView layer ] shadowOpacity: 1.0 ];
  </code></pre>
  
  <p>If using shadow effect causes bad performance seriously, I will draw shadow of the images directly.</p>
  
  <p>If there are some tips about this issues, please let me know them.</p>
  
  <p>I want to use shadow effect on iOS programically, because I have the worst drawing skill.</p>
  
  <p>Thank you for your reading.</p>