068f2f15_Improving_Drawing_Performance__improving_drawing_performance_Tip
[Action] During each update cycle, you should update only the portions of your view that actually changed. If you are using the drawRect: method of UIView to do your drawing, use the update rectangle passed to that method to limit the scope of your drawing. For OpenGL drawing, you must track updates yourself. [Tip] 
Draw minimally
Improving Drawing Performance