Well, it would help if you told us the rest of you PC's specs, as I'm guessing the issue is there, as all the issues people are having is with the game's graphics, NOT performance as far as speed goes. It's more than likely you don't have enough Ram, or you don't have a good enough processor if you're experiencing that level of lag.
Windows 7 64bit fresh install today. Only the game and all the drivers installed, no other programs. My specs as follow:
OS: Windows 7 64 bit
CPU: AMD 1090t overclocked 4Ghz
Ram: 8 Gb Adata Gaming series V2
Graphic Card; XFX HD Radeon 5850 1gb of ram
Hard Drive: WD Caviar black 750Gb
Everybody with an ATI card is having issues after the intro cinematic. I can play crysis 2 without problems, can play The Witcher 2 all options set on Ultra without problems, but cannot play this "awesome" game made by Carmack