While there is no “one answer fits all”, the majority of employment arrangements are at-will, meaning you can fire employees for any reason or no reason whatsoever.
However, there are some legally prohibited reasons. Union workers may have protection within their contracts. Also, employees critical to infrastructure might have protections due to the nature of their work. i.e. transportation workers critical to keeping shipping and transportation moving. You must also consider any limitations that may have been agreed to in an employment contract or agreement.
The challenge is for an employer to actually have proof that a worker is intentionally slowing down and isn’t just hampered by a disability or lack of training. The same issue arises when an employer has to prove that an employee is intentionally calling in sick or has a legitimate health issue that flairs up periodically. Have regular conversations with employees about work, production and quality are essential to have baselines related to non-performance or lack of performance issues.
Because employment law can be complicated and every situation can be unique, contact your business legal counsel for specific issues or laws related to your situation.