had the same problem with one i purchased this weekend (dont worry thekid it works now)
put in a few different lots of batteries, and still slow.
when i got home, i opened it up, freed all the wires, put the board in my other halo to make sure nowt was wrong. everything seemed fine.
Put it all back in and it works fine. Might be a trapped wire? Also i found that the small piece coming out the top of the motor can rub against the shell and slow it down. I Wedged a some folded up duct tape between the blue thing and the shell and it stopped.
Try having a look and cleaning. Probably something simple