What can a PI do about a low-performing postdoc?
This really depends a lot on the contract situation. In some countries, such as Germany, there is a “probationary period” of several months wherein either side can choose unilaterally to end the contract. This lowers the risk from a bad hire. Saddled with a hire gone wrong, I’d try to get what output I could from the postdoc during the period and start a new hiring process right away (or see if anyone from the previous search was still available!).

In countries like the US where contracts may not include probationary periods, you have to try to figure out how to best help the postdoc out, since you’re stuck for a potentially much longer period. If it’s some sort of missing knowledge you figure out how to provide that. If it’s some other issue, you try to work around it or solve that problem. Without a more specific example, it’s hard to draw up a good “plan of action.”