I tried just outfitting a MULE and running around the unoccupied land and found no slow-down on the mountains. I'm running the latest Mac version. (Update: Tried it without a MULE in tow too and no slow-down on mountains that way either.)
Also, I appreciate the thoughtful response regarding last round auctions. It did strike me that this kind of "departure" from the classic game might be a good fit for your concept of switchable on/off new features.
Dan