Legal Question in Employment Law in New York

You've answered your own question -- in New York, employment is "at will." That means that either the employer or the employee can end the employment relationship at any time, for any reason, even a bad reason, or even for no reason.

The only exceptions to that are if the employment action violates a written employment agreement (such as a collective bargaining agreement) or constitutes unlawful discrimination (based on such factors as sex, pregnancy, age, disability, national origin, color, creed, race, or religion) or retaliation.

The fact that you are "performing at 100% and actually excelling at the position" offers, unfortunately, no legal protection.

Good luck to you.