Hi,
Just wanted to chime in. I too have the steam version and was nailed by the massive framerate drop. Here's everything I've done:

1)Had great framerate up until I “beat” the level as a keeper. I experienced no slowdown, but I was pretty much just focusing on my one sprite and running around with him a single turn at a time.
2) Framerate drop happened when I released control and went back to my lair. On screen there were anywhere from 25-45 sprites. Drop went from 40+ FPS to 0-5. Usually this was a full lock up for 1-5 seconds followed by 1 or 2 seconds of play, then a lockup again.
3) I saved the game, restarted, and reloaded. (just in case of a memory leak) This did nothing to solve the problem.
4) Came to the forum and read this post, went to the steam forum and followed the advice about the cores. Any single core affinity didn't work (was maxed out on the performance chart), frame rate was still low (1-10) with full stops. When I went to 2, or 3 cores, framerate was stable  around 22-37, but always 22/22 or 37/37. The one thing I did note was that as long as core 0 wasn't selected, this was the case.

Hope this helps. Here are my computer specs for future info:
Processor Intel(R) Core(TM) i5-2500K CPU @ 3.30GHz, 3301 Mhz, 4 Core(s), 4 Logical Processor(s)
Installed Physical Memory (RAM) 8.00 GB
Video AMD Radeon HD 6800 Series