To adjust the speed of the animation, enable “Time Stretch”
Right-click on the layer and choose Time → Time → Stretch
To calculate the percentage you need to use the formula 100 / x where x is times slower. Lets say you need to make the video 3 times slower, 100 / 3 = 33.3. If you just need to make it 20% slower, you can deduct 20 from 100 and enter 80.
You can also click and move to the right or to the left.