068f2f15_Improving_Drawing_Performance__improving_drawing_performance_Tip
[Action] By default, UIKit clears a view’s current context buffer prior to calling its drawRect: method to update that same area. If you are responding to scrolling events in your view, clearing this region repeatedly during scrolling updates can be expensive. To disable the behavior, you can change the value in the clearsContextBeforeDrawing property to NO. [Tip] 
Avoid clearing the previous content during scrolling
Improving Drawing Performance