I've read that there is an app for that. Seriously. I think it may be called App Killer? Something along those lines.
It seems there are real battery issues if you don't actually close out of apps you've been using, as they continue to run in the background, chewing up your battery. I'd think they would slow the system down too, but don't know. The app somehow helps by closing apps you're not using.
I have yet to pick up the Eris, but plan to, thus have been researching. If you can't find it, try Googling 'HTC Eris' and check out other forums. I've seen it stated on a number of occasions. Hope this helps.