EVO 4G LTE
With the blessing of the OP on another forum I'm copying exactly what he posted. I have done this today as my phone was done charging 100%. I'm 4h 3m later and the battery is at 92% remaining. Based on my battery life over the last more than a week now(13-17hrs), I'm very confident I'll get 20hrs on this charge. WiFi speeds are not effected by this setting.
WARNING: DO NOT TURN ON THE "Best Wi-Fi performance" setting!!! On the Evo LTE, it will DESTROY your battery life by consuming tons of CPU and keeping the CPU awake all the time, regardless of if you are using the phone or not. This has nothing do to with other settings such as "Keep Wi-Fi on during sleep" (which I have set to "Always" with no problem). I have tested this several times now, and believe this bug has probably bitten other people too.
With the setting on, you will notice a battery drain of 200 to 300% faster. Under power-> battery use you will see unusually high "Android OS" percents. When clicking on that, you will see high "CPU total" and very, very high "Keep awake" times..... in fact, the "Keep awake" time will almost mirror your on-battery time.
After searching high and low, I can find no definitive explanation of exactly what that "Best Wi-Fi performance" setting really does, anyway. The word "performance" is meaningless- could be bus speed, chip speed, negotiated speed, battery impact, awake times, connect times, transmit power levels.... just about anything. Hopefully HTC/Sprint will fix this or at least disable the option if it cannot be fixed.