Great input @Paperbot, we really appreciate your ideas! Sorry for my delayed response, got hit with the man flu.
I wrote a bit about the slider slowdown in the issue Misaki linked above (fixed in 4.3.25-beta, available now). We put reasonable effort into the editor being efficient and occasionally we do testing with huge projects to see where further improvements can be made. For this, sliders are new and were doing work that could be avoided. Still a draw order timeline with 26,000 keys is relatively slow in the editor (27fps for me). Probably don't do that.
The idea for "clear" keys is a good one! 4.3 might be a good time to get it in. We'll check how disruptive it will be to implement.
The idea for numbered animations for sliders is maybe a bit complex. We have considered allowing a slider's animation to be keyed. We even implemented it, but removed it. For sliders to work significant runtime changes were needed (that's why porting is taking a while) and this feature complicated that. The dust is mostly settled now, so maybe it could be revived. When a slider animation changes, the skeleton "update cache" has to be recomputed. That is relatively heavy, but could be OK if it doesn't happen very often (surely not 26,000 keys, right?!). The editor already does it every frame so the impact is only at runtime.
Anyway, once you can key a slider animation, then you can just use another slider and that gives you the numbered slider animations that you wanted. There is no problem that can't be solved by throwing sliders at it!