Very interesting thing is that in Dear Esther (game using portal 2 engine) I have the same problem.
I tried to reinstall steam, reinstall portal 2 itself after that, I tried to change video settings, I tried to turn off some aplications like Skype or Anti-Virus tool, I tried to update my video card drivers, I tried really everything- no effect. Still lags.
I really don't know what I should do, does anybody have an idea?????
PS (Because of Lpfreaky90 reply)- IT IS MEANED THAT I'm puting it into sleep or hibernation mode while game is NOT running.


you could make a screenshot of all processes just after a clean boot from off and see what processes run; save them; hibernate/sleep your computer; boot it and see if there are any new processes: maybe they cause your problems?