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if thats the case u do might want to check your computers for viruses and stuff like that and btw, you may also have other pgroams running slowing the crap out of your game. the best thing to do is close all programs

Well, your computer may not have enough space to run Left 4 Dead. Having other games such as Call of Duty on your computer takes up enough space already, and Left 4 Dead is just very slow due to that. You can fix this by deleting a game or some files that you don't need. Even if you have the best computer around, or just a good computer, it can still happen just for the fact that theres too much stuff on your computer. My computer can't hold Garrys Mod, in this case, it may be the same for you, exept your problem is Garrys Mod.