Yeah, after thinking about it I came to the conclusion that this was your use case, replaying a high-speed video at its real time speed.
Currently there is no satisfying way to do it. Tweaking the reference video framerate in order to increase the range of the speed slider is a workaround based on a side effect, I don't think it's a good solution to overload this option with a completely different role than what it was intended for.
The issue is that if you want to replay a say, 1000fps video to real time with the current player, it will need to decode and display these 1000 frames per second, it will most likely fail, which to me means that this use-case requires a different playback approach altogether.
I very much like the idea of being able to go way higher in speed. If we can do 10x we should be able to do 100x because at that point it wouldn't be related to computer perfs.
This would be very useful to quickly parse a long video, and it would provide timelapse out of the box which I think have unexplored applications in sports, to analyze cyclic motions or long term trends in posture.
So, I would say: 1. a new playback approach for very high framerates, most likely by seeking forward in the file instead of decoding frame by frame. 2. a logarithmic speed slider so you have fine control for slow motion but can still go to 100x speed at the other end, 3. an increased size of the speed slider (this is limited by the size of the screen in dual playback configuration at the minimal monitor resolution supported).

