I have the same problem, but I just don't know why.
PC specs:
MBo: Intel H67 USB-3
CPU: Intel(R) Core(TM) i7-2600K CPU @ 4.00 GHz
RAM: 8 GB (2x4GB) DDR3 1333MHz
GPU: nVidia GeForce GTX 550Ti GDDR5 1GB @ Stock speed
HDD: 1TB WD 7200RP/M
PSU: 600W Corsair

It's so annoying, that I'm recording on 1080p in Assassin's Creed, with 60+ FPS, and when I turn Fraps on while playing LFS, and start recording, it decreases dramatically.... about 10FPS...
Tried to take down anti aliasing, and the sky drawing to 16 bit, but it makes only 5 FPS difference between the two settings...