This is what they don't tell you:
(1) The workmanship is shoddy. True, the damn thing goes up in a weekend or two. But they are flimsy. Workmanship is poor. They take shortcuts to get the building up on schedule. Just watch and see how long it takes before the roof starts leaking, windows (if any) start getting drafty, or the climate control system conks out or goes berzonkers. I wonder if the constructions will last nearly as long as some of the slow-build halls of the past.
(2) Most of the actual work was done before. They pour a slab, often months before the building goes up. Much of the plumbing and incoming electrical is done at that phase.
(3) Prefab sections are often utilized. Even if the walls are done on sight, with 40 people working on a wall, it can be assembled (rather sloppily) in less than an hour. Simultaneously, they have other crews working on other walls and the roof trusses. I have seen home improvement shows, and given the number of people working on the hall at once, a weekend or two sounds about right.
(4) Who's going to pay for it when the congregation dwindles? I have heard threads on this board where congregations have been in a pickle because they do not have enough funds. Window washers can only give so much, and that is often not enough. And, if most of the congregation goes apostate, there is not going to be nearly enough money to pay those mortgages.