Lag: If you are getting a 'laggy' (unresponsive, or delayed reaction) feeling when playing Crysis, consider the following: in almost all cases lag is caused or exacerbated by low framerates. In either Singleplayer or Multiplayer, check your FPS using either the r_DisplayInfo 1 command, or FRAPS, and if it is often dipping below 15-20 FPS, or even more, especially during heavy combat or around certain special effects (e.g. ice), you will usually experience some lag. The only solution is to lower your settings and adjust command variables until your FPS rises.
source: TweakGuides.com - Crysis Tweak Guide