Hitman Contracts Post Filter

In Hitman Contracts, I notice that when I enable the Post Filter setting,
even on Medium, framerate drops a LOT. In areas where I would normally get
60-90fps, I get around 15-30fps. I guess it enables pixel shader 1.1
(according to the readme). Is it nomal for such a framerate loss. Is pixel
shader 1.1 new? I've never seen it b4.

With Post Filter disabled, I can play the game at 8xAA, 8xAF and it runs
super smooth.

Barton2800+
MSI 5900FX 8x
Nvidia 56.72
Asus A7n8X Deluxe
1GB PC3200 Dual Channel
WInXP