For the best performance in editor play mode, make sure the scene window is behind the game window. If you don't need to view the scene view gizmos, you can disable them in the Preferences window. Edit/Preferences/Simple Traffic System
Often resetting the unity window layout to default (top right corner of Unity Editor) will fix editor related bugs. Unfortunately this is an unresolved Unity bug that has been around for a long time.
When cars are spawned, the spawn point's collider bounds are checked to make sure it's empty. Ensure there are not any other objects touching the spawn point colliders.
Please see this forum post for a guide on installing unlisted packages in 2020.1 or 2020.2: https://forum.unity.com/threads/simple-traffic-system.794268/page-6#post-6318243
AITrafficCar will set the car’s top speed, each AITrafficWaypoint will set the car’s speed limit.