Sounds like you might have something running in the background slowing it down. Did you try the battery saver tips in the "Battery Life" post? That turns off 4G, WiFi, Bluetooth which can keep scanning in the background.
Also if you don't use your mail servers alot, set them for manual push or set the push time for longer than a few hours. I know with my Hero when it would slow down, just powering it off would sometimes help (closed running background programs). I've only had my Evo a few days and have only had one slow down and that was when it was low on battery and I have too many tasks going.
Check your battery consumption too to see what is using the most power consumption.