Biggest Speed improvments on ipad and iphone4 in general (doesnt affect 3gs too much):
- dont use opengl 2.0
- dont use shaders using transparancy of any kind
- if you do use transparent something keep the as small as possible on the screen
- dont use blob shadows (as this will result in transparancy calculations)
- dont use lensflares (transparency)
- use mobile shaders (you find some in standard assets)
the polygoncount itself is not so important on ipad/iphone4/3GS