When does it work fast, and what do you do before it begins slowing down?
And what exactly is slowing down?
After using AE my computer begins to slow down. Once that happens I try to move the mouse around which is also very slow.
Wow. That's a bit hectic.
What version of Windows?
GPU Drivers up to date?
AE version reasonably up to date?
When it slows down, are you able to jump into the Task Manager to see what's eating up the resources?
If not, run task manager in front of other windows and try provoke the slowdown.
AE can often bog down the hard drive on machines with less ram than the project requires, as it caches a lot of stuff. I had same issue until installed an SSD for AE cache