If this started happening recently, it is because of the beta that is going on and the number of background updates that the game is going through. The beta that they are testing is mainly about changing the engine that the game runs on, which is much more intensive than the previous Source engine. I noticed this also and I went searching on the Steam Forums and people were discussing how the non-beta users were getting drops in performance. I used to be able to max out the game with my native resolution but now i have to have everything on low to run my native resolution at a steady 40 FPS... The update will be publically released in the next month or so and performance will this increase to where it once was.