This hasn't happened to me in either of the first two episodes. The game starts out playing just fine, but after a while (maybe 15 minutes or so, I'm not sure), it becomes extremely slow and Windows has a message about virtual memory being too low, and the CPU usage is at 100%. So then I have to exit, let it "cool down", and start again.
I have 512 MB RAM. I know I should get more (I had more on my old computer, although everything else about it was worse), but the game starts out just fine, and episode 2 didn't have this problem. So it seems like a bug.