Hello, the idea is that the speed of the spawn belt should be higher or equal to any other belt it feeds into, otherwise the belt will not be fully saturated.
What is the reason you would want it to be slower? Visually?
If you want less items to come out of it you can just put a slower belt in front of it.
As far as I know the belt speed can not be dynamically adjusted, so it has to be one speed.
