The onboarding cliff is the week-six drop-off: the warm welcome is over, the documentation has been skimmed and forgotten, and the new hire is now quietly expected to just know things.
It is not a talent problem. It is a support problem. We hire well, hand people static docs and a “good luck,” then act surprised when capability falls off a cliff right when the real work starts. The fix is unglamorous: keep the practice going past week one.