self.layer.contentsScale = [UIScreen mainScreen].scale;
    self.layer.rasterizationScale = self.layer.contentsScale;
    self.layer.shouldRasterize = YES;
    self.layer.shadowOpacity = 0.7;
    self.layer.shadowOffset = CGSizeMake(0, 0);
    __self.layer.shadowPath = 
[UIBezierPath bezierPathWithRoundedRect:
CGRectInset(self.bounds, -2, -2) 
cornerRadius:self.layer.cornerRadius].CGPath;__
    self.layer.shadowColor = [UIColor blackColor].CGColor;
    self.layer.shadowRadius = 4;

More Detail : Animation's performance is very poor when view's shadow is on

http://stackoverflow.com/questions/7746921/iphone-animations-performance-is-very-poor-when-views-shadow-is-on