I've been on both sides of this equation and in NYS, at least, one cannot be fired for 'performance' unless the employer has met at leasttwo standards...
1) A record of your poor performance and the employer's efforts to correct it or at the ;east warn you about it must preceed any dismissal.
2) Every other employee of the company with the same job must also be shown to meet the standard that you failed to or else be fired too.
Absent these two conditions NYS Dept. of Labor will represent the employee in a wrongful discharge suit (as long as the employee is not a 'manager').
If they have either sat you down to discuss the performance issue prior to this and/or notified you in writing that your performance is an issue then there is little (probably nothing) you can do except take the money and wave good-bye.
Excellent post. Saving it. May need it in the future. The 2 witches in the office are still trying to get me fired.