I'm not sure, but I believe it has something to do with the
resistance of the extra-long cable.
Electrical resistance is directly proportional to length. So
less current ends up going to the cable, and the battery takes
longer to charge up.
Bottom line: the shorter the better. Just stick to 1 m USB
cable.
Eh Fudgey... I think he meant the 3M certification, not the
length...