How to slow down animation of a Group?

I’ve already created a smoke simulation (without relying on particles, meaning there’s PLENTY of keyframes), but it turns out that its speed is just too much. Somewhere here I read that a “slow-mo” is possible for certain groups in Synfig itself. How do I adjust the speed of this animation of a certain group without manually working with each keyframe?

Group everything in a Group layer
Set the Time Offset parameter to linear
Then it offers two sub parameters:
Rate & Offset
Setting the rate to -1s would stop the animation. Setting it to something between -1s and 0s would slow down it.
if rate < -1s then it is reversed
if rate > 0s then it is accelerated

It works! Awesome! Thanks! That feature seems as hidden as a control-room in a James Bond movie. xD

Just one problem: while it “slows down the animation” alright, rather than stretching keyframes it seems to play as if the animation is on 3 FPS…

EDIT: Just tried with -23f (with the project being at 24 fps). It literally is at 1 FPS animation…

Added the trick to the wiki