My old setup was Ubuntu 10.04 LTS on an AMD Athlon II dual-core with 4GB of no-frills DDR3 RAM, and a Geforce GTX 465 with 1GB RAM, all on a no-frills motherboard. Even after following the performance improvement thread I could never get above 13 FPS in Meridian, with everything set to Low.
My new rig is Ubuntu 10.04 LTS on an AMD Phenom II X4 Black Edition with 16GB of Corsair XMS DDR3 2000 memory, a pair of Nvidia GTX 580 OCs with 1.5GB RAM in SLi mode, and an ASUS Crosshair motherboard. On High settings with all shadows, projected textures, and the full screen glow turned off, I'm getting a whopping 21 FPS in Meridian, and still dip well into the single digits when moving around the courtyard area on my low-level mount.
My wife has an employer-provided Windows 7 laptop with 4GB of no-frills RAM, a no frills Intel mobile dual-core processor, and a dedicated Nvidia GTX 470 with 512MB RAM, and on Windows 7 she gets about the same frame rates that my new rig does.
Yes, I know that I should expect some performance degradation due to the non-native OS, but something is incredibly wrong with this picture. This isn't a little bit of degradation; it's an exponential curve.