if you've got 2008, right click and hold over the timeslider and a little meu should pop up, go to playback speed, playback max realtime, or realtime that should solve it, or as Neo said a playblast always plays back at realtime.

BUT

If your animation uses dynamics they wont calcualate properly as it needs every frame, so then drop it to max realtime or if its too slow then you'll have to do a playblast.