[MonoTouch] iPad app built with mt5.0.3 runs fine on ipad2 but very slow on ipad3

[spcware]

looking through my code, I think it just might be my lazy inefficient quartz
2d graphics code. I think I am doing a lot of full screen repainting when
only a small portion of the screen has changed. 

that got me rethinking my code and now I am stumped. what is the best way to
do fast 2d updates where only small portions of the screen change?  based on
my research, iOS doesn't support nondestructive drawing. every drawrect
command will clear the entire screen before executing the drawing commands
regardless of the clearcontextbeforedraw property setting. therefore, the
only way to update a portion of the screen while keeping everything already
on the screen untouched is to use layers. that is what I am doing now but it
still very slow. even with using layers, you still have to draw that layer
on the graphics context which still repaints every pixel contained within
the layer whether it changed or not.  I must be thinking of this
incorrectly. 

imagine the entire iPad screen broken up into a 3x3 grid such that when you
touch any of the cells, it changes color. what is the best way to implement
that functionality?  thanks for any help. 

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