Slows down when? When you preview? Do you know how to preview?
If you are pressing the space bar to preview the video then the behavior you are seeing is normal. AE will not playback in real time with autio unless you do a ram preview (look it up). You must learn to use the software.
Start with the three videos on this page: Basic Workflow
Or check out these resources: AE Basics
Trying to play H.264 in realtime? There's another issue.
After Effects is huge. Unbelievably monstrous.
It is the single most terribly complex and yet admirably versatile and still weirdly intimidating software application you will ever try to use.