That's a bummer dude - I really wish they'd be more specific about the defects. Saying you have a performance issue really isn't explaining anything, which makes debugging almost impossible. Did they say what areas were slow, and quantify what 'slow' means? Low fps, cpu usage etc, and under what sort of hardware?
Also I do remember reading somewhere that it's recommended that we run the WACK on old PCs as well, as it may fail there due to lower specs.
Anyway, don't give up hope just yet, give it a good try - if you fix it in today or tomorrow and then submit it, sometimes they can turn it around really quickly on the same day. That's what happened for my 3rd submission.