just stopping by from the tech support forums, this has to be bug related but many including me are having problems getting any steady fps in coop mode, every time a player comes onscreen with a few skellys popping out, massive fps drops (from a smooth 60 to 1 or 5 or 15 for a few seconds)

i am on a i7 @2.8 and an ati 6870m 1gb, and for me my fps was fine at 1920X1080 at med/high settings single player. lowest settings, no shadows, low fx, coop at 1600X900 is unplayable. i have the latest drivers i can get from dell

Does that matter? I can't even start the game and have like uber long game launching time.
But like you asked, I clicked on play button before the download is completed.

It matters because streaming the game (playing when the patcher is in yellow) is not as optimal of a player experience as waiting for it to finish patching.