Nobody can tell you anything without exact system info or other technical details. Aside from general differences in how AE treats this stuff, a lot of this depends on the actual configuration, i.e. preview settings, audio preferences and all that stuff.
Here are the specs for my Macbook Pro:
On AE, the preview is set to real time and quality set to Quarter. And as for memory utilization goes, I reserved 5 GB for other apps to use and the rest should be graciously available to After Effects.
So, I just find it odd that the same files without applying any effects whatsoever still chokes AE. I believe Adobe Premiere also stores/caches data in the memory or HD and it also playbacks the video in real time. But don't know why AE chokes. Maybe it's encoding first and then playing from memory.
To contrast the issue, I tested the same project on Camtasia Studio 3 and NUKE. No issues there :/ .... Perhaps AE engine is not as efficient as the Premiere one.
You must be new to After Effects. The playback engine is completely different. AE, even with the improvements, must render everything before playback. Premiere Pro uses a modified media player for playback so most things are in real time and if PPro has to drop frames to keep up with real time audio it does so without even letting you know that it has dropped frames.
The fastest way to work with After Effects is to have your composition panel magnification factor set to 50% or less and have the comp resolution set to Auto. This will give you the fastest previews. It's also important to realize that After Effects is not an editing app. It's primary function is to produce shots that need motion graphics or compositing that you cannot do in a NLE. More than 90% of my comps are less than 7 seconds long and involve only one shot. A lot of my comps are only a fraction of a longer shot in an edit that need work. For example on a movie that I'm working on right now there is a shot of a cowboy getting on a horse and then getting shot. The entire shot is just over 4 seconds, but the part where he gets shot is only 22 frames long. My ae comp is only 45 frames long. It starts 2 frames before the bullet hits and extends beyond the out point in the first cut by a few frames to give the editor some room to adjust the timing of the cut in the final edit. That is how you efficiently work in AE. Process just the frames you need to process.
I hope this helps. Slow previews and short previews in AE are a fact of life probably for at least the near future. They are working on improvements and completely rebuilding the back end of AE but don't expect it to be as user friendly as a NLE or even much more usable as a tool to edit sequences for at least the next few versions.