Are you new to AE? Are you doing a ram preview? After Effects uses an entirely different method of playback than a NLE or a media player. Sometimes it takes a while for your system to catch up.
We can't know any specifics unless you give us details about your project, your system and your version of AE down to the last decimal point. Most often folks are trying to preview long segments of an AE comp at full resolution (resolution never needs to be higher than zoom factor). I have mine set to Auto in the comp window and usually have the zoom factor at 50%. You are probably just over taxing your system.
Try purging your ram (file-purge-ram) This causes the video to render faster. Otherwise, look for the green bar above your timeline, the green represents what is rendered and what you can watch at full fps, if it is not green, let it render out and turn green and then go back and watch at real time.
Hope this helps.