Not an answer to your question, but the Sagrada Famila [1] began construction in 1882, and is projected to be completed around 2030. It's an immense (and beautiful) structure. Just want to point out that it's not just a matter of the buildings being built a long time ago; this kind of long-term construction is still happening today.

That said, Christopher Wren was in high demand for designing churches after the Great Fire, So he made a deal: I will design you a church but no steeple (yet). Once the backlog of churches cleared he designed the steeples, and some churches had a 20 year gap.

Even pre-industrial London could crank out enough churches in a dozen years to keep a celebrated architect booked solid. But that’s an exceedingly dense urban era, capable of supporting a large array of skilled labor.

I... just can't wrap my head around this. Is it that they actually take that long with enough people, or they have too few people working on them?

They're specifically using old-style techniques to build it, and doing a sort of fund-as-you-go structure where donations finance the construction. This is combined with the fact that the building is really complicated and every block has to be custom-carved into a unique shape.

There's also likely an element of not really wanting it to be done since the construction itself is a big part of the draw of the building for tourism.

And yet the author of the present article would simply judge it by weight and say "with certainty" it must have taken only a fraction of the time.

I hadn't heard of it, it seen pictures, until we went to see it in Spain. The building is absolutely mind boggling. The sort of thing that makes you proud to be a human.

When you see it, the timeline starts to make sense.

Funding but also they are to an extent a giant work of art, adding more people does not produce a better work of art.

Funding has been the limiting factor, as far as I know. There have been long breaks where no work was being done, and it was damaged during the civil war. Also the building is immense and very detailed.

The fact that Gaudi's original plans were lost in a fire don't help

Some cathedrals took centuries to complete, but mostly because of long gaps in active work due to lack of funds or interest.

Furthermore cathedrals were often build iteratively - e.g. someone finances a new tower to add to an existing centuries old building.

You might enjoy Ken Follet's novel "The Pillars of the Earth", it touches on the notion of spending a lifetime to build a cathedral.

Ah, thanks for the pointer! I just Googled and got this explanation: https://www.quora.com/Why-did-it-take-nearly-200-years-to-bu...

You run out of money, wars happen, corruption happen, things get finished later.

I mean, look at Petra. Some of that stuff must have taken centuries.