Couple of things:
1. Highly recommend you do NOT edit footage in After Effects. It is not built for that sort of thing. Premiere Pro is built for editing. After Effects is built for doing motion graphics, visual effects, and animation. Basically, it's for doing Effects After the edit. 😄
2. After Effects doesn't play back a video stream like an editing application does. It fully decompresses the video file to show you fully accurate pixel information. That is, it procces and then plays back every pixel in every frame. So, it has to cache a preview before you can plan it back in real time. The issue you're likely facing (without more information, this is my best guess), is that you're just watching it while it's caching the preview. While it's playing, you should see a green line appearing above the current time indicator in your timeline panel. The green areas are the cached frames. If you press 0 on your number pad or Shift+Space while it's building a preview, it'll play back through the frames that have already cached at real time. (Assuming everything is at the default settings in the preview panel.)
Does that help?
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