title: CGContextDrawImage very slow on iPhone 4
body: <p>I use CGBitmapContextRef for large alterable image.
  Than I draw one's parts in UIView.drawRect.
  That's work well on iPhone 3GS - 17FPS. 
  But on iPhone 4 calling subj method is very slow - and total FPS is 1.5!</p>
  
  <p>profiler says that: on 3GS invokes CGSBlendBGRA8888toRGBA8888 which takes 23% of total time. on 4G same code invokes argb32_sample_RGBA32/argb32_image_mark which takes ~90%. wow!</p>
  
  <p>Color space for bitmap is CGColorSpaceCreateDeviceRGB. And creating flag is kCGImageAlphaPremultipliedLast.</p>
  
  <p>What can I do for better performance on iPhone 4 ? Any suggestions ?</p>