Demoplayer is used to view Q2 (and some other games) demos just by double clicking a demo file. You can watch demos fast forwarded or slow motion. Demoplayers default config contains only 6x fast forward, so here is alternative config for 20x or faster playback. Install it to Demoplayer folder. For smoother slow motion use AprQ2.

Demoplayer itself can be downloaded here. Install it anywhere you want.