Maxing out on thresholds of metrics

Let's assume you have far crossed the threshold n-1 metrics but your nth metric is good.

Now doing any worse with the nth metric would have a catastrophic impact on the PSI score even if you marginally improve other n-1 metrics.

You can experiment with the Lighthouse Score calculator

Try to improve any metric other than CLS(which is already good) and try to increase CLS value.