my ssd is 91 percent full.
If this is your root drive, then 91% is enough to be considered full.
There's no rule of thumb but the constant opinion is that your root drive should never go over 60-70% full or performance will degrade.
This is only info I've read over the years about Linux.
Is it true or not?
Maybe some more expert opinions will give a better insight about this.
This is just one info I read in 2013:
How is the minimum file system free space decided? (linuxquestions.org)
And following it:
Impact of space usage on file system performance - Blue Light - Blue Light Wiki (bluelightav.org)