Why you need more space than you need!
I often talk with consulting clients whose very legitimate complaint is that their hard drive(s) are just too slow. Sometimes it’s also a “stack 'o drives” mess— an unruly pile of cables and drives with crappy power supplies that will soon fail (get the QX2).
But sometimes it’s just because they are using Firewire 800, an idea whose time has long passed (and a major reason the iMac is “dead-end Mac”).
But often it’s simply because they’re running with a drive that is 70%, 80%, 90% full— so it’s running at about 60-70% of its peak speed.
Hard drives slow down as they fill up. Read more.
With constant data density, and π X diameter determining “data velocity”, the speed is a simple matter of bits traveling past the heads of a hard drive.
But the wrong choice of SSD slows down for other reasons, and some are “dogs” to start with, so caveat emptor: SSDs are not inexpensive.