Like your phone is slow, or it sucks too much battery. Just not behaving like it should.
First you try with Soft reset: Just out with battery (or press power button for about 10 seconds if you have one x and such, most sense 4.x phone), let it rest a while. If that does NOTHING, do proceed with the solution below.
Not that this is the end all solution to all problems that you can encounter when doing a major update on your phone. But, it will give a better start with the update. And, the solution is to simply do an factory restore. Just backup your contacts and stuff and your back up and running in no time.
Sense 3.0 phones
Settings > SD & phone storage > Factory restore
Sense 4.0/4.1 phones
Settings > Storage > Factory Data Reset
Settings > Backup & Reset > Reset phone
If you need more help with you phone, there’s plenty of guides and help on the HTC help pages