• This bug is fixed in Unity 2021+.
• If you're using a Native Animator Controller you can control its overall speed by setting the Animator.speed (usually accessed via the AnimancerComponent.Animator).
• But if you're using a HybridAnimancerComponent or ControllerState it's not possible to directly set the speed.
  • The ControllerState class has a Speed property Inherited from the base AnimancerState but Unity's internal AnimatorControllerPlayable simply ignores it.

Workaround

Other than updating to a newer version of Unity, there's only one known workaround (and it's not very good):

1. Create a Float Parameter.
2. Select all of the states in the Animator Controller.
3. Enable theirSpeed Multiplier and set it to use the parameter you just created.
4. Repeat steps 2 and 3 for each layer and sub-state machine.
5. Now you can use ControllerState.SetFloat("Parameter Name", speed); to control the speed of all states.

Unfortunately, that only affects the states themselves, not the transitions between them.