Everyone has potential to turn it around and get awesome at their job. This leads to the hand wringing and delays he is taking about. But if you are hiring senior leadership with experience ... You probably expect they have figured out a lot of their work practices already. Other commenters are pointing out that juniors typically take time to flourish and that's normal.

Lars is focused on the fact that keeping a senior employee who gives you a bad feeling that things are not working out is actively harmful to your company. Whether they turn into a rockstar later in a different context that big of a deal if you replace them with somebody who is at lest not causing good employees to leave.

You'll probably never know if you fired too soon. Confirmation bias Will confirm the decision. You'll always know if it was too late.

isn't it always "too late"? The fired employee would have to have a long pattern of problems. With the benefit of hindsight you'd know that you should have fired them after the first 5 or 10 problems.