I've got huge FPS drop in linux 32bit build of "Unigine Heaven" benchmark when I setup my system to use dual-screen with Xinerama. FPS drops down to ~15-20. Switching back to TwinView dual-screen setup or using single-monitor setup fixes the issue - FPS goes back to ~60-80.
AMD Phenom II X4 955 CPU, 6GB DDR2 RAM, nVIDIA GeForce GTX 550 Ti with 1024MB VRAM, two monitors with analog interfaces, nVIDIA 32bit Linux driver 280.13 (also tested with 285.05.09, affected too; I had to downgrade driver down to 280.13 version in order to workaround another regression in 285.05.09 I had reported earlier).
Unigine Heaven settings:
Windowed 1024x768 mode, AA disabled, trilinear filtering + 8xANISO, tessellation disabled, low quality shaders, high quality textures, ambient occlusions disabled, refraction and volumetric shadows enabled, vsync disabled.
This bug also affects Wine, reported here: http://bugs.winehq.org/show_bug.cgi?id=27959
Wine developers believe that FPS drop happens due to nVIDIA drivers bug in GL_ARB_map_buffer_range extension that behaves really slow in case Xinerama is used.