A new patch was released last week for Stalker Clear Sky. This patch (v.15.08) greatly improves performance, especially when antialiasing is enabled.
http://cs.stalker-game.com/en/?page=patches
The new patch also adds a new form of ambient occlusion called “Horizon Based Ambient Occlusion.” HBAO produces a softer and more accurate rendering than the default screen space ambient occlusion (SSAO).
HBAO is not enabled by default in the game. To enable it, add “r2_ssao_hbao on” to “user.ltx” located in “C:\Users\Public\Public Documents\Stalker-STCS”.