Usually my game frame and draw time are between 8-12 ms on the servers, but it is possible to make these much worse via alt-tabbing during loading screen. Then I have to play with 13-19 ms.
I was easily able to test this while loading a map locally. For example obj_trainyard:
^ performance after the bug has been triggered
^ normal performance
Once you have triggered this performance problem, you need to restart the game to fix the issue. Going back to main menu or loading another map does not fix the issue.
To trigger it, you just need to alt tab. Just try to alt tab during loading screen as aggressively as you can and you should be able to trigger this.
Please note that I do NOT refer to the performance hit that we experience after tabbing back into a game. That one usually fixes itself within a minute. The one I am reporting seems to be permanent.