Non-Linear Editors and After Effects handle video in completely different ways.
With your setup, it seems quite normal, yes.
AE is not intended for real-time playback. You render a portion of the animation/composite out and watch that.
You really shouldn’t press your spacebar to preview (I’m not sure why that’s even included as an option) Press 0 on your number pad to initiate a RAM preview, you will see green start showing up on the top of your timeline, these are the rendered frames. Since you don’t have a lot of RAM, I wouldn’t expect it to give you a terribly long preview, but that depends on the resolution of your comp. It will either hit the limit of available RAM and start playing back or you can press 0 (on the number pad) again to play back real-time.
See “Preview video and audio” for more info.
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