Is this an instance of "workplace discrimination" based on a disability?

If it's discrimination it will be hard to prove, since concerns had been expressed to you regarding your performance prior to you revealing your health condition. Even if you have a disability, though, that doesn't excuse you from performing to your employer's satisfaction--all it entitles you to is "reasonable accommodations." If you believe that certain reasonable accommodations may help you improve your performance, you should make your employer aware of this fact. As you have already been put on a PIP, your time may be limited. http://www.johnphillipslaw.com

There is no way to know whether your employer is discriminating against you because of your disability without more evidence. What you have now is mere speculation. Employers certainly have the right to expect their employees to meet productivity goals and you indicated concerns about your work were expressed by your manager before he or she could have known about your "long term health condition". If the PIP was related to the expressed concerns, they have a defense to your suspicion. Once an employer becomes aware of an employee's qualified disability, it has an obligation to engage in an interactive process to determine what can be done to reasonably accommodate the employee's disability so that the employee can perform the essential functions of their job, as long as the accommodations do not cause an undue hardship on the employer. The critical issue for you is that you still must perform the essential functions of your job. Having a disability is not an excuse for not meet your goals, in most circumstances. You have the right to write a factual rebuttal to the PIP, if you do not agree with it and ask that it be placed in your personnel file. And if your health condition is affecting your ability to do your job, you have the right to ask for a reasonable accommodation so you can perform your job properly. If in doubt, seek the advice of an experienced employment law attorney in your area.

Consult a local employment attorney who can review the facts with you and advise you as to your next steps. Good luck.

An adverse employment action taken against you (which a PIP may or may not be depending on the facts and circumstances) which is taken due to your disability, would be illegal and form the basis of a disability discrimination claim on your behalf. However, as was mentioned here by another poster, your facts are such that bringing such a claim might be difficult, as the performance issues were raised with you prior to your notifying your employer of your disability. That said, if you believe there is evidence to support the idea that your employer brought the PIP after learning of your disability, then you may have a claim.