It's a pool divided up among all players based on playing time and salary.
Each team gets an equal performance pay pool to divide up between their players. It is divided based on what % of their team's plays each player participated in, multiplied by the inverse of their salary. So someone with double the salary as someone else would get half the bonus for the same playing time, and someone with twice the plays as someone else with the same salary would get double the bonus. (At least that's how they explain it in a simplified way, maybe they tweak it more than that, I don't know.)
One of the odd things that gets lost in the publicity about people like Koppen getting a $100,000+ payoff is that everyone gets such a check, but most are relatively small. So Brady will get a performance check this year which could well be the lowest on the team.