I'm inclined to believe that measuring performance is fairly subjective. However, in an attempt to put some numbers behind a performance score, you might have some combination of individual weighted scores that involve things like:

(1) length of time employed at the company

(2) some measure of the performance feedback the engineer receives in their annual review - which may include percentage salary increase (possibly subjective, though)

(3) a score compiled by surveying the employees' peers (again, possibly subjective)

(4) the overall TripleByte turnover rate at said company

(5) the market average for any/all of (1)-(4)