I picked up Necrovision during the summer sale, and I've been playing it for the past few days. I just got to Chapter 6, and I've noticed that the Vampire SMG is firing a lot slower than it's supposed to. It's not the whole game slowing down, just that weapon. I have no idea what the problem is. I'd appreciate some help.

That's weird. I'm having an issue with the game as a whole stuttering and slowing down while the weapon fires at its normal rate

It full on crashed my PC! i uninstalled after that. A janky fps with a great gimmick isn't worth wrecking my rig. Would like to know if anybody got this fixed though.

Apparently turning on V-sync causes fps issues in the game engine (in this case the "Pain Engine"). I recommend turning off V-sync and see how your performance is after that. I, personally, do not own the game BUT I have been doing my research on it as I want to purchase it. It is the spiritual successor to Painkiller: Black Edition after all. And that's one of my favorite games.
So yeah, V-sync causes issues with Necrovision and its stand alone expansion pack - Lost Company. Therefore turn off V-sync.