Found this info about servers on a L4D forum today no more lagging servers:)

If your console isn't enabled yet do this

Goto Options > KeyBoard > Advanced > Enable Developer Console.

Then press either ~ or the ` key

Then at the main menu hit your console key (I'm partial to ~)

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