There are 2 possible reasons.
One is your computer cannot play the video fast enough as it is under-powered but most modern computers will have no problems just playing video in Pro8.
Another option is you are using a high speed video.
What does this mean? Depending on your video format (NTSC or Pal), Pro8 plays all videos back at 25/50 or 30/60 frames per second (fps) respectively.
If you have recorded from a device that captures at greater than 50/60 fps (e.g. high speed video camera or iPhone 5/6) the action in the clip will appear be in slow motion at a playback rate of 100%. If you wish to view it at a faster speed move the rate slider up.
To know exactly how fast to play it you will need to know the original frame rate. For example if it was filmed at 100 (NTSC) or 120 (Pal) fps you will need the playback rate controller at 200% to make the action in the video look like it is at 'normal' speed.