I Have bad performance on using shadow effect

I put some image views on scroll view. And when I drag this scroll view, I didn't have any problems.

But after I applied shadow effect to these image views, dragging this scroll view has bad performance.

I used shadowOpacity, shadowRadius and shadowOffset methods.

ex:

[ [ anImageView layer ] shadowOpacity: 1.0 ];

If using shadow effect causes bad performance seriously, I will draw shadow of the images directly.

If there are some tips about this issues, please let me know them.

I want to use shadow effect on iOS programically, because I have the worst drawing skill.

Thank you for your reading.