> I found that one a new team and they're very productive again
This. I should have added context that my experience was in small companies where it wasn't really possible to change teams. But some of the people that were bad performers became good performers only after they were fired and found new jobs. I've seen this happen at more than one company. They were probably just demotivated or hated their boss or something, and no amount of intervention can really fix that, short of a change in job (or a change in team, as you mention, if it's a bigger company).
Some people don't know others see them going slow and managers don't have conversations till it's become problematic. Some people don't realize or won't inculcate feedback until they're fired. They play brinkmanship with their managers. Those people firing cab help. Kick in the pants as it were.
This. I should have added context that my experience was in small companies where it wasn't really possible to change teams. But some of the people that were bad performers became good performers only after they were fired and found new jobs. I've seen this happen at more than one company. They were probably just demotivated or hated their boss or something, and no amount of intervention can really fix that, short of a change in job (or a change in team, as you mention, if it's a bigger company).