068f2f15_Improving_Drawing_Performance__improving_drawing_performance_Tip
[Action] If you are calling setNeedsDisplay:, always spend the time to calculate the actual area that you need to redraw. Don’t just pass a rectangle containing the entire view. Also, don’t call setNeedsDisplay: unless you actually need to redraw content. If the content hasn’t actually changed, don’t redraw it. [Tip] 
Call setNeedsDisplay: judiciously
Improving Drawing Performance