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.