You may have a bad app running in the background.
I haven't had a task killer since I had my old Hero. And I have never had a problem with lag.
One thing I do know is that Live Wallpapers adds a generous amount of lag.
Try the same test, but without live wallpapers, and see if it turns out better.
Another thing, the reason why most people say to not use ATK or another task killer is because most people don't set permissions for ATK to not kill certain process's, like Message, Phone, Clock.
Killing those process's can make a SMS message come in hours late, and can make your alarms never go off.
Some people have different results with different methods. Just use what works best with you, that's the wonders of Android. You now have a choice!
Welcome to the community, and I'm glad you like Android of Iphone.
Lg Fusic>Lg Lotus>HTC Hero>HTC Evo