Your computer reads the directory of all volumes into memory at startup. For every file it reads the information of the first 4 fragments. Every time you want to access fragments after those 4 fragments, it has to read this info from disk instead of memory. Then it starts to work like a Winblows computer and you lose a big advantage of Classic Mac OS. This problem goes away by defragmenting, also on SSD. Don't say that an SSD is fast enough and that you won't feel the difference. Every HD/SSD can do only a limited number of asynchronous calls per second.