Well I don't think it's a matter of people in the city it's more a matter of how many of them are actually building. And in most 4x games the building speed is based off some production amount in the city. However in elemental the buildings all take a fixed amount of time to build as long as you can afford them. Also buildings are resticted by city based on level so one could potentially argue that the reason they can't build it is there is not enough people to support it.
Most 4x games since building production rate is based off some production value they have the late game buildings cost more production. That way less developed locations take so long to build some of the late game stuff it's fairly impractical for them to build. Rather then population they could use level as sort of a production indicator. Say each level produces 1 point of construction and thus a level 5 would build 5 times as fast as a level 1. But of course the higher level buildings would have higher production to keep them in line with how long it takes to build. I recommend using levels cause it'd be hard to use pop and balance 1 pop city vs 1K pop city on construction unless you had some overly complicated formula.
Honestly I've never been real fond of the your city must be this level to build this building. I like the approach of other 4x games where it's like "Sure you can build that uber wonder in your new found city. But it's gonna take you over 100 turns." I think it would be more interesting if they make it so you can build all of the buildings regardless of level but tack on the construction modifier thus a level 1 city would take over a 100 turns to build something that is now level 5 resctricted. As well as just increasing the build time of stuff in general, except those 4 core early buildings.
I suppose the reason I'm not fond of the city level limits is it really penalizes you for research high up the Civics tree early on. I mean most of the early techs I get require level 3+ cities with a few level 2 items in there. It's very easy to unlock stuff you won't even be able to build for another 40+ turns which I think is a terrible design of the tech tree. While the other trees you can use most of the stuff right away as long as you have the resources and the high city level requirement stuff tends to be buried deeper in the tree.
I recall in Beta focusing mostly on Civics because of all the stuff there was to build. Now thanks to the fact that cities can only build 1 of each building type, which was a good change, the result is cities with nothing left to build that sit idle. And almost no interesting choices on what to build since your level bound on stuff and frankly most stuff builds way to fast.
I have played a few games of elemental now and I don't know what it is but the game feels like it flies by in terms of tech and city development. I know it other games like GalCiv and Civ it felt like I was going at a snails pace at times trying to get the latest and greatest tech as well as build up my planets/cities but in Elemental everything seems so fast. Oh and I play on Normal speed.
