I think it really depends on the SSD as well. Some have lots of memory on board and/or over provisioning. Others have no RAM or little extra space. Those that have their own ram and extra room won't need to have 20-25%+ space free. While those with no ram and no extra space need as much room as you can give them.
I could be wrong but the type of SSD cells might also play a part. Many TLC and/or QLC drives run slower once you fill up their first layer of cells. Once you use up the faster SLC "Buffer" they slow down a lot. To my knowledge there is no way to make it run fast again until you free up the space. But a lot of this depends on the drive you have.