We have fast forward to speed things up, but currently, no way to slow them down (other than outright pausing).
It would be very trivial to implement this. On the other hand though, I don't know if it'd be worthwhile even for a trivial feature, given that we have pause and framestepping anyway. But I'm open to implementing this if anyone thinks it'd be useful. (IchoTolot, for your solution videos, perhaps?)
I envision this being about 50% of normal speed, but other options can be considered for sure.