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.