I asked some questions to better help me answer your question. I asked those because they are legally significant.
I'll try to answer without the information I asked for.
In employment law, your employment is either under contract of some sort or it is not. Most of us (and that is by a very large margin) are uncontracted employees, also called "at will" employment.
In an at will employment situation a person can legally be terminated at any time, with or without cause. If that is your situation, then you really have nothing to lose by signing a performance improvement plan. The employer doesn't need you to sign it to terminate and if you refuse, they can just terminate you.
The only times you would not want to enter into a PIP agreement or would want to negotiate a better one is if you are a contract employee, with an agreement stating you can only be terminated for cause, OR you suspect discrimination as the basis for the request to sign.
If you have any further questions, please let me know. I invite follow up questions, so use REPLY for those. If you have no further questions then good luck going forward and please do not forget to rate my service with a three, a four or preferably a five star rating so that I receive credit for working with you today. Please rate me based on my service and not on your satisfaction with the law, which I am not in control of and I am just reporting to you. Also, feel free to request me in the future, if you have questions concerning a different matter.