I have an ATI card too, and for me the switching time was much shorter after SR1. You can disable "enable hardware shading" in the "Preview renderer" section of the Render settings. This did the trick for me before SR1. The "hidden Line" mode should work quickly as well, but you won't see the shaders then.

From what I've heared, the time is needed to build up the scene for the graphics adapter, so clearing the render cache is unlikely to help. 

Did that work for you?

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

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