PROTIP if your AirPort Extreme has abysmal performance regardless of position or settings try giving it a factory default reset.

I have one that has gone through probably 4-5 in-place firmware upgrades over the last few years and somewhere along the way started to provide horrible downstream throughput to clients. No amount of troubleshooting on my part could get it to work correctly. After giving it a reset its back to working as expected – yay for WiFi faster than dial-up.