3- Yup, nailed it, same reason WMA and Flash require "activation" on the PSP and why newer MPEG codecs like H.264 and AAC require an "optional media update" on the Xbox 360

2- The slowdown usually comes from scanning the FAT to count the free space, with the PSP being notorious for this

Another related point is the varying support for the "semi-official" 64K cluster size, which gives a curious effect on the 3DS: in GBA mode (officially only used for 10 games only distributed as compensation to those who bought the original model at full launch price), a strip of "junk" (coming from the virtualized GBA's VRAM, IIRC it's just wraparound) will be shown at the bottom of the screen...