Playback will be choppy but once you produce your timeline, your final output playback should be okay. Say you speed things up 10x, that means you are only playing ever 10th frame of your original file so you need to process through all the frames in your 8 min file very rapidly during playback, that creates the stuttered playback. Once you produce the file, playback will be smooth as every 10th frame is now consecutive to create the speedup you wanted.

If you want, you can move the orange timeline markers around the area and use the PD "Render Preview" feature to pre render your accelerated area, playback will then be smooth in the timeline. This takes time to pre render, little green bars appear above timeline indicating progress and when completed remain to highlight the fact that this area was pre rendered.

Jeff