[gvitaly]

Not all players are eligible for Performance Bonuses, and to receive a bonus within their contract, a player must meet one of the following criteria:

1. The player is on an entry-level contract.
2. The player has signed a one-year contract and is over 35 years old.
3. The player has signed a one-year contract after returning from a long-term injury (has played 400 or more games, and spent 100 or more days on the Injured Reserve in the last year of their most recent contract).

Also because the cap hit of a contract is an average value it would give teams a way to circumvent the cap, which is already done to an extant.

Performance bonuses count against the salary cap; however, a team can exceed the salary cap due to performance bonuses by the maximum performance bonus cushion amount of 7.5% of the upper limit.

EDIT: ^ What he said.