The need to onboard new hires is one of the most significant reasons why adding workers to a project does not actually increase the project’s productivity. It takes time to get them up to speed with how the system works, what their role is, day-to-day workflows, and a host of other things. The goal of any new hire, even ones where you aren’t trying to fill in gaps during emergencies, is to get new hires as productive as possible as quickly as possible.