I was on one of these performance plans at my last job. What I learned from my experience - once you are on a plan, there's rarely any coming back. It's a road to the end, how quickly that happens is up to you. The microscope is on you and they will find any fault to make you want to leave. I was also not busy for a lot of my plan period, because the work was going to other people instead of me, to increase the feeling of worthlessness (although I was happy to get paid to do very little). My mental health and confidence in my work took an absolute battering. I'm glad you've decided to move on.