You should NOT expect AE to playback video or audio in real time. It just isn't capable of it for more than 20 or 30 seconds tops depending on what's in your project. You would think it should be able to, but it isn't. To make things better you can make sure you have plenty of RAM and fast dedicated cache drive that is not full. Also in the preview panel make sure you have "Cache before Playback" ticked.
But yeah, unfortunately AE struggles terribly to playback in real time even with a very fast computer.
It's just the way it is and there's no reason to believe it's going to change.