I've recently bought Overgrowth and played Reciever that came with it, but like many other people I experience gigantic lagspikes and low framerate during gameplay. Forcing my resolution to something low like 800x600 kind of works but the game looks at it's worst that way.
Now, one thing I noticed about the game is the fact that you can shoot down lights and lamps. This means that all of the light objects are dynamic and put a major strain on your processor.
Sadly I'm no modder (yet), but I ask those who are to try and make that light static/baked light instead of dynamic and see how that works.