Congratulations Nanaa, you now have the unofficial motivation to one day impress drk with a Savage video, since this first-time video passes only as rank 'Ok'.
Yea slowing down the demos with left and arrow keys is one of few replay commands for demos I'm glad that works. One of Tirza's videos showing XR gameplay with a Predator and Legionnaire in particular being spun around in super slow-mo (to the point that the clips looked like it was paused gameplay) got me searching for it awhile ago.
Did you also know there's a command to manually set (read: fast-forward) to any time in a demo? I think in SFE it was '/time 0.00' or something simple like that, should be the same with XR -- I know clan commands were changed in XR, but haven't heard of the demo/replay console commands be changed.
That might help you speed up to stuff you really want to cut out for your videos, this'll save you from having to record the entire demo and then cutting up one large video file -- if you got one specific moment of gameplay from a demo you want to grab then just use /time 0.00 (or plop in the time you need to jump to) and you can pinpoint the clip you want and be back out with single clip(s) yer want. I'll bet oschores dating bearloths that at least drk, Butterfly, and Tirza/H3027 use this technique for their videos.