I don't know the answer to this question, but I was under the impression that hard drives got slower as you filled them up because the heads would have to move across the disk and start filling programs in the nooks and crannies that were free between other areas of the disk that were taken up, causing more head movement and thus more time seeking.
Not sure if SSDs have the same kind of problem or not, or perhaps it's just a thing that happens as the OS tries to cope with such little storage, not sure! Would love to see someone more educated on the subject chime in here.