Measuring individual performance, not just that of testers but of any role, is extraordinarily hard and fraught with peril to badly distort things. Bad actors will always game the system–“Look at the 1,253 bugs I filed on improper punctuation!”. This is part of why over the years I’ve come to really dislike (OK, outright hate) using metrics like number of test cases written/executed, number of bugs filed, etc.
I’ve long tried to use “up one level” metrics as part of any review or performance process. By that I mean looking to how the individual helps the team succeed, or even better yet how the organization is better.
Some examples I’ve used include number of support tickets filed 5/10/30 days after a release, number of renewed licenses, etc.
Those things are HARD to measure. Which is why a lot of organizations simply cop out and take the really awful approach of using bad measurements instead.
As testers we need to be part of changing this! We should raise the bar about the value we truly provide to the organizations.