There isn't any real objective way to measure performance though - there are things you can quantify (lines of code written, code reviews performed, etc) but those don't encompass the entire job, and once it becomes clear the metrics you use people will work to optimize their numbers at the expense of the other aspects of the job that are harder to quantify/measure. Who has time to mentor junior colleagues when you gotta bump your LoC count for the quarter?

I think we need to embrace the idea that whether you are given raises, promotions, continued employment, etc is largely a function of whether your manager likes you, likes having you on their team, and wants to reward you. And if they don't like you, it doesn't matter how good your numbers are now they will find a way to get rid of you sooner or later. It's better to just get it over with, give the employee a generous severance package, and part ways (or transfer them to another team that's interested in them).