I'm not really sure where you get this information from. but it's not really true.
it really depends on what data is on the disk, how it's arranged on the disk (how fragmented if it's a mechanical disk), how much of it you're accessing, how many times you're accessing it, whether it's the system disk, how big your page file is for example and whether that's grown to it's max size., how much space you're allowed for temporary files...
So many variables.
Feasibly you could have 99.99999% used, maybe even 100% used and the computer could still be just as fast as if only 10% was used (in some circumstances.
the max percentage also depends on the size of the disk.
1% free on a 20GB drive is only 200MB, 1% free on a 1TB disk = 10GB free