Well, just out of curiosity, I defragged my K3 (er, Kindle Keyboard).
It was 29% fragmented after I guess almost 3 years of use. Took surprisingly long to defrag, almost 2 hours.
I want to say it feels quicker after I defragged it, but it could be my imagination. Should have timed things before I did.
But here is a test someone did on an SD card
http://www.lagom.nl/misc/flash_fragmentation.html
Quite a noticeable difference - more than twice as fast to read