Storage, what percentage to leave free? Flash memory and SSD controllers are reported to spend more time running wear-leveling and/or anti-fragmentation garbage collection algorithms when their storage is near full.  

People have reported that clearing some free space on their near full SSD storage, and leaving their iMac or MacBook running overnight (to do something like wear-leveling or storage block garbage collection) increased storage speed/bandwidth performance significantly.  Similar things are reported to occur involving UI lag on iOS devices that have almost completely full flash storage.

So, is there a recommended percentage of free space to leave on the SSD on a Mac, or the flash storage on an iOS device, to avoid having these algorithms kick in and possibly affect storage speed or read/write performance.
I read that things slow down when it is less than 20% free on a SSD or HDD on a Mac. I noticed my read/write speed seemed to improve when I freed up some space to get near the 20%.  I don't know about IOS devices but 20% seems reasonable there as well.