Performance drop for people with Multicore ATI GPUs/SLI Nvidia and 64Bit OS.
Every time the map changes, performance for people with multicore ATI GPUs/Nvidia SLI GPU's and 64-bit OS's drop to unplayable levels
Possible fix : Alt-tab out of game, then back in (original post)
Possible fix 2 : This one is for Nvidia users. If you are running driver 195.xx, try downgrading to 191.07 WHQL. Note this does not mean it will be fixed, but is a possible temporary fix. Thanks to Beowulf_ .
Solved by valve : Unconfirmed