Mine is on 3.5 days at 17500 each too. I have kept constructing the tiles since it was first introduced, and now I have a pretty nice sizable boardwalk.
Yes, the boardwalk tiles are deliberately to be built in long hours, unless u like to speed them up with your precious donuts.
EA knew we are stockpiling in-game money, and that we are able to construct our Squidport quickly if the tiles can simply be purchased instantly, and we may be soon ended up complaining nothing to do next. So EA started this prolonged way of earning each tile, as an incentive for players like us to have something to look forward to, and continue to login to the game even without event.