This article is good but imho misses a big potential issue: the manager’s perception of performance.

Managers, especially newer ones can have lots of biases. Someone you think is underperforming may actually be doing ok, they’re just not approaching the work in the way or at the exact same performance you or superstar Sam does. Not everyone is a rockstar, and if you insist on that you are going to have trouble scaling.

So my first question is “how is this manager evaluating performance?”

Follow ups -have those expectations actually been communicated to employee? -what specifically has been done to help the employee? Is it lip service or actual help? Has HR and managers manager been looped in and do they all agree employee is underperforming?

Only then do I think you should move to reassigning / firing