Task Manager

No you shouldn't need one. If it makes you feel better go for it. I used one for 3-4 months with my first Android phone, then removed it, and didn't notice a bit of difference one way or the other.

 

Don't use the task manager. The slowdown on your screen is probably because the task manager is killing some of the processes linked to your homescreen (widgets, etc). So every time you use one of those processes or swipe across the screen those processes have to be reload. You're making your android do double the work