Hi Pete and thanks for reply.
Indeed, this must be some sort of graphic driver issue. I tried installing different versions of Catalysts, including 9.3 which was supposed to handle OpenGL correctly, but no luck. But once I switched to my onboard GPU (HD 4200) the framerate is good.
Do you plan to add support for DirectX at some point? It could be helpful since problems with OpenGL in Vista and Win7 is a common issue.
Anyway, thanks for help!