I've been having the same problem as you. Unfortunately I also get unplayable FPS when using swiftshader.

But swiftshader isn't bypassing the pixel shader, its just using your cpu to power the graphics as well and unless you have an amazing cpu the fps wont get better.

What I was hoping would be possible would be to use swiftshader to get past the warhammer system check, but the be able to disable the pixel shader so my fps doesn't blow so bad.

Anyway please let me know if you figure anything out, I'll do the same.