The only map you can play in tf2 is dule dule because you have realy bad frame lag on big maps all your settings are low and you have this faint brown fog im pretty shure thats the reason for your lag

It depends actually. It could be 1 of 4 things
# Your latency - A classic case of not having fast enough Internet speed. To fix this, you'll have to get better internet or get a better router. You can also close all browsers before starting TF2
# Processing overload - Another classic case, this time you don't have enough power in your computer to support TF2. If you have been able to play other maps other than duel, than this is not what's happening. # Dirty computer - This is very common. If this problem has just starting occurring, than this is what is happening. This can happen when you have a virus or a fragmented hard drive. Most of the time it is caused by registry errors. These errors are can easily be fixed with a registry cleaner. Here's the one I use: http://www.eusing.com/free_registry_cleaner/registry_cleaner.htm # You could have modded the game too much and have caused errors.