You've given an excellent answer to the question Why is Dropbox's LAN Sync so slow? but the more interesting question is Why don't the Dropbox engineers change how LAN Sync works so it won't be so slow anymore?
Also, uploading the data to Dropbox server, and then downloading again, wastes valuable quota for those under a data cap.
Do you want Dropbox to make a ghost entry with the contents and the hash, then start transferring it over LAN right away? That's going to be confusing when a ghosted file doesn't behave the same as a normal unsynced file. Especially if you start doing collaborative work and the version history is broken because that's cloud-based.