Another longer term idea, input welcome 
Today the range of values for the speed slider is mapped like this:
With a logarithmic scale, it could be like this:
Advantages
- more precise in the slow motion area of the mapping, where it's most important.
- can go up to high values without eating too much space.
Drawbacks
- less natural changes when manipulating the slider ? (there are no tick marks on the actual control, only the current value is displayed).
- too much space eaten by very low values ?
Going up to 10x (or more) would be interesting for two features: 1. playing a high speed video back to normal speed, 2. playing a long video ultra fast to quickly scan for interesting sections. Technically, to keep the pace, instead of trying to play every frame, we would jump and skip as many frames as needed.
I'm not sure about the final displayed value though. I think for slomo "10%" is better than "0.1x", but for high values, "10x" makes more sense than "1000%". Mixing the two is not entirely satisfying for some reason.
A more advanced mapping function could be experimented with maybe.