Check out this link for L4D lag/performance fixes. The Nvidia driver issue is particularly key if you're an Nvidia user. I was suffering from a big lag problem with the latest drivers until I went back to the 178 drivers as they suggested.
http://left4deadforums.com/1873-lag-sound-loop-crashes.html
Getting Low Ping ServersAlso, the tip for forcing L4D to only connect to low ping servers is particularly useful too. I used
sv_search_max_ping which worked great.
QuoteLag:
*Note This can't always be fixed due to location, and ISP
At the main menu hit your console key (Defualt ~) Then in the console type in "openserverbrowser" (without the "s) and hit enter go to the custom tab and re-fresh all and join a server that pings less than 100 to you.
At the main menu hit your console key (Defualt ~) Then in the console type in sv_search_max_ping 100 (default is 250) Should make Launching games from the Lobby a bit less painful.