A judge would automatically throw out the case if this was the argument for suing an employee. The reasoning being, if you continued to pay the employee during the term of their employment, and you knew that the employee was not performing based on some KPI or some yard stick, you would issue warning to the employee to improve their performance, or you would fire the employee on the spot. Continuing keep an underperforming employee is giving tactic consent that their work is reasonably acceptable because if it wasn't, you would start disciplinary action or cease their employment.
Threating a employee with coercive threats (such as threats of legal action) is going to land the business into hot water in any modern society.