Does anyone have advice on handling poor performance with employees?

This is a delicate situation. What performance conversations have you had to date? Have you followed the progressive discipline approach? Is it documented?

If you terminate the employee without these steps, it can look like it was due to disability. Of course, a discussion with an employment attorney is always advised.

Before you proceed, is the employee asking for an accommodation for the disability? Is that what is affecting performance? It may not just be that the work is challenging due to a physical or mental condition – getting to work on time might be at issue if the employee needs to make alternative travel arrangements.
If the disability itself is impacting performance, you’ll want to work with the staffer to try to make an accommodation. Without more specifics, it’s hard to give direct advice, but depending on the size of your business the Americans with Disabilities Act may be applicable (15 or more full-time or full-time equivalent), in which case you’ll want to make every effort to make an accommodation as required by the law. Even if the disability has no bearing on performance, it’s a best practice to try to accommodate the staff member as much as possible: if all efforts fail, then meticulously document where accommodation efforts have been tried and failed, and where performance issues have not been corrected before moving forward with dismissal to assure you have tried every possibility but are left with no choice but to separate the employee. Good luck!