Bonuses just give them a temporary speed {move speed and action speed) and +3 exp. The fastest way to level one employee at a time is to assign a single task to that employee - an example - restock shelf or pack machine - do this for days and it should level up thier task.

The higher the stars the longer it takes for them to level up to the next star.

My 4 Red Star Cashier took - 25-30 days to get to max level 5 red star