If I have my alt on autofollow and mounted, then stop on my main. If I try to scoot up a little closer on my alt while he is still on autofollow he moves very very slowly. Unmounted he can move normally but mounted he is slowed. Its been like that for maybe forever.
Try putting a "/run" command on the second line in your auto-follow macro. I've done this for all of my characters which has helped in some scenarios. May or may not help your specific case, and I wouldn't be surprised if some cases are permanently problematic.
