Hello,
I needed this feature in my game and i realized this changed a little since last version of Godot Engine.
On idle games, strategy game, time scaling is required. You can find answer for question how to scale time in Godot Engine below:
Engine.set_time_scale(scale: float)
If you set scale to 2.0 time will pass 2 times faster, if you set to 0.5 time will slow down half.