I've mostly been running the game in XP mode on my laptop. In OS X mode I got some pretty severe slowdown. When my frame rate dropped to 3 fps right as a zombie started growling at me is when I switched. The problem is definitely worse in multiplayer - in SP I get crashes here and there, but not the insta-gib I get in multi.

I'll try playing multi in the browser and see if that helps.

Welcome aboard Denis!