If you are finding that the speed is a little too fast, here are a few ways that you may limit the amount of power and speed outputted by the system.
1. You can lower the top speed of the bike to under 20 mph. Please note that this will affect only the pedal assist speed of the bike, you can still go faster using the throttle.
2. You can adjust the MAX PAS parameter to 9 levels of assist. That way there is significantly more variability on how much power the motor is outputting. Levels 1,3, and 5 are speed limited by default when operating under 9 levels of assist, so this should help in travelling a little slower.
Here is a video documenting how to make these changes: