Hello there!
I’ve been trying to find a way to temporarily change the playback speed in order to hear some music written in Allegro vivace much slower to hear all the details. I don’t want to change the tempo of the piece, but just the playback speed temporarily. Kind of like a rehearsal tempo 
I’m not sure if this feature has been incorporated or is somewhere on the wish list for future updates… Or maybe I just can’t figure out how to do it…
You can adjust the tempo for the “Allegro vivace” mark in the Properties panel. Just remember to change it back afterwards.
Or you can slow down the tempo in payback mode in tempo panel. And again delete it there after you have finished listening.
I figured just as much, and as of now, it’s the only workaround I’ve found. But it would be really nice to have an easier, integrated, way to do it
Having two separate controls that govern the same thing is bound to cause problems. You’d get loads of people posting here about why playback is at the wrong speed, when the Tempo Properties pane says it’s right.
Sibelius has a temporary playback panel …
It would be nice if Dorico had something similar in its playback panel - it could always flash a pulsating pink to remind you that you had reset the tempo temporarily!
I agree with rodrigoruiz that it is very useful to check out fast complex passages.
This has been a feature of Sibelius for years and I don’t think causes issues of confusion (their Tempo Slider) - when it’s in the middle, the current playback tempo matches the actual metronome tempo; when it’s to the left it plays back slower than the indicated metronome tempi, to the right faster (etc). Very handy for precisely the reasons the OP stated (especially in a piece with multiple tempi). Logic (and other DAW’s, I’m sure) also has a similar feature (Varispeed, where you can specify to slow down or speed up the entire session by up to 50%, including audio files, without affecting pitch)…
Some key combination to temporarily 1/2 the speed would be very cool.