Thanks for that tip. I disabled the limiting now and it sped up from 11:42 minutes to 01:40 minutes! Now Dropbox is only x12.5 slower than AeroFS.
The real solution, of course, would be for Dropbox to transfer the file over the LAN immediately before waiting for it to be uploaded to the Dropbox servers.
That's not a solution, it's a design tradeoff. Dropbox's servers are the "truth", and the software's clear goal is to get a copy on the servers. There are many reasons for this--weird conflicts, inconsistencies, and so on--that are much harder to resolve than if there is a copy on the servers.
Once that happens, other clients will ask to download from the servers, notice that it's on the LAN already, and take it from there. I bet if you install a new client on the same LAN, or share a folder between clients on the LAN, or add a folder to selective sync, once the files are already there... it will be a competitive speed.
The real solution, of course, would be for Dropbox to transfer the file over the LAN immediately before waiting for it to be uploaded to the Dropbox servers.