The best hires I've ever made have started submitting code reviews within 24-48 hrs of being onboarded. Sure it's not for something like Microsoft Windows, but still this has been a hallmark positive indicator of a good hire for me, and applicable to many (but not all of course) frontend, backend, and app development projects.
Now maybe there is some discussion to have about what "100% efficient" means. Of course they will not hit their peak of productivity on day 1, and anyways if 100% efficient means the most productive day they've ever had, then they will more days than not be < maximally productive.