Hughe performance differences on different resolution

[Myxir]

I just was testing with several resolutions in windowed mode. I discovered that 1024*768 in windowed mode gives about +50% FPS compared to 1280*1024 in windowed mode.
That seems a very big difference to me. I never saw such a big difference in another game so far.
1280*1024 in windowed mode with disabled window decoration gives me ~30 FPS on max settings which seems a bit low to me. Changing from highest to medium (including AA and Anisotropic filter) brings like +5 fps only.

System
Ubuntu Karmic 64 Bit
AMD Athlon II X3 435 (2,9ghz)
4 GB Ram
Nvidia GTS 250 (195.36.15)

Can somebody confirm or prove me wrong? It seems a bit odd to me.