Megabyte (MB) and a Megabit (Mb). Mb being 1/8 of MB, so 1MB takes 8 seconds if the speed 1mb/s.
From memory the class of an 03ds sd slot is 4, n3ds sd slot is 10 (no uhs). That is your limit on the 3ds device regardless of the sd being higher, however copying files from a pc to the card i.e...