I see that like
@aieeegrunt , but you're right, it doesn't make sense to go through the whole process again in other cities, unless those cities are disconnected from a City that has a given infrastructure. Once a Project of a Science District has been succesfully completed in a City, it should be much easier and faster to do the same thing in other cities that are connected to it, be it through Road, Trade Route, or unlocked with a Tech/Civic/Policy.
This can easily be done in a new Civ Game, however the Issue with City Productions/Projects that always bothered me in the Civ Games, is that cities always have to work on something, which on its own isn't the Issue or unrealistic, but in Civ VI, a City ALWAYS HAS to be working on something, and that something is usually a Building or a Unit, occasionally a Project. I think a redesign of this Concept could be the Solution to "unrealistic building Time for Buildings and Units", with Units training taking 10s or 100s of years and Wonders 100s of Years.
What if we make the production of Infrastuctures just like discussed above and with Units training taking a max of 3-5 Turns (mostly in the late Game, because of Years/Turn difference from early Game) and once a City has built all the infrastructure it can build and doesn't have to build Units, the Player can simply ignore building something in that City? we could have the same amount of production Choices that we have now (Districts, Buildings and Units), but with much more Projects to work on. We could have Projects that improve certain Buildings or the Benefits of a Tile or the Citizens who are working the Tile (like training them so they can be more efficient), Projects that help other Cities building their stuff (like help building a Wonder or producing certain parts of a Units components (imagine a City is building a Ship and the other is focusing on training the Naval Units for that Ship)). Another Idea is making Unit Upgrades only possible via a Project in the City where the Unit was trained.
This way building something in a City would be much faster and the City would still always have something to work on (if the Player wants to), is it either improving something in that City, working a new Project or helping another City accomplish its Task. And if nothing special can be produced in a City, the Player could simply command the City to maintain itself, which increases Stability, and perhaps also the Yields generated (efficiency).