The latest OTA release of Gingerbread for the HTC EVO 4G introduced more than a few new problems. One you may not have noticed yet is that if you have an 802.11n wireless router, you're now only connecting at 802.11g speeds with 802.11g range. In other words, you're not crazy. Your WiFi speeds are slower now because the update killed wireless N support.
Fortunately a fix for some rooted Gingerbread Sense ROMs (with HTC #1 kernel) has been cooked up and is available in the form of a flashable zip at the link below. Make sure you make a Nandroid backup as MultiDev (the author) suggests before attempting it, and don't forget that this is only for Sense-based ROMs. If you try to flash the fix and you've got a custom kernel and/or an AOSP ROM, not only will you not get your wireless N back, you'll completely break your WiFi as well.
If zip flashing isn't your thing, MultiDev also provides instructions on how to manually replace the existing driver with a modified one that includes N support.
Unfortunately this will not work on an unrooted EVO since there's still no method of rooting the OTA Gingerbread update. So those who are on stock OTA Gingerbread will either have to wait until Sprint rolls out another maintenance release or someone manages to root the current build.[xda-developers]