My Nexus 7 also has really poor WiFi performance.  It has gotten to the point where I don't even bother trying to use it anymore.  It works well if I'm in the same room as my router.  But my desktop is that room as well, so I might as well just use that. I've barely had it a day, and already it's become nothing more than a paper weight.
Aside from this thread, I haven't been able to find many owners complaining about WiFi performance.  So we might just have defective units.  I think I'm gonna call customer service to see if I can get a replacement.