Only large groups of people with low outdoors who used taxi alt will travel slower. They can use car instead of a lame alt. Or make more balanced characters, not powerbuild apes moved around by proxy char-vehicles.
They will just use more taxi alts, as one taxi alt won't be enough for getting a group of people to destination. At least, maybe number of "players" in game will get higher.
I hope you don't expect that due to your brilliant idea how to nerf taxi alts, "pvp apes" won't find an easy workaround? Or do you? Oh one more thing. "Powerbuild apes" already mix using taxi alts with cars, so probably groups of newbies travelling together will be the only people slowed down.
Also, where's the sense in slowing down, let's say, 10-people group with one 300 outdoorsman alt?
Ever heard of a guide (person)?
So now, what does make sense and what does not?