Huh, when was the last time you played? I had a lot of slowdown on release but the latest patch clears it up. Maybe an issue where the game doesn't like your hardware or something? Unfortunate though, it really is a fun game and the team has put a lot of time into improving it that they didn't need to and didn't get paid for.
This game has never, ever run perfectly for me. Runs good up until the finale....and then its a lag-crushing exercise in futility. I've tried the game on two different rigs, the second one almost tripling the overall power I had (from a single core to a quad core, double the memory, x2 the video card...)
And the finale still runs like garbage. The whole trap building/barricade thing? A complete waste of time. The first wave alone takes them all out. The game is a ton of fun right up until the end...where even after several patches, it plays like crap. I can't even imagine what the finale would be like without 100 zombies running face first into the very first wall of the main room. I couldn't carry enough ammo and weapons to finish them all, and relying on the AI is pointless.
Sorry for the minor derail, but I think you're the first person I've ever read say that the game plays great all the way through. I'd agree the game was fantastic and worth playing if the ending pretty much never existed. Unfortunately, it does and the whole game builds up to it.