I personally feel that the X-Ray Engine is also pretty unoptimised. I play Crysis Warhead at Enthusiast Settings, 1280x1024, 4xAA and frames are around 40-50. At Gamer, they're about 50-60. On very few occasions do they drop to 30 and lower.
However, I get about 20-30 frames on trying to play Clear Sky with all details set to the highest. I have to sacrifice on the lighting effects to be able to play without dropping frames.