I love that so many Civ Fanatics now see the truth of this original balance. It's interesting to see how much of a difference Magnus can provide in a city- often I can see 30-40 extra production in his city. (Disclaimer: I build IZs everywhere.) This is important to keep in mind: that's a 20-40 production boon in every city we had access to under the original rules.
But towards the OP,
Mines are phenomenal improvements. In fact, Civ6 makes hills the optimal terrain, because hills yield the normal amount of food. With the changes to farms, a player can generate crazy amounts of food on a small plot of flat land anyways.
A developed late game city usually sees 2-4 big sources of hammers:
-mines
-t3 gov't bonus
-maybe magnus
-maybe trade routes if you stack a few in one city
Let us dig into the first two. Mines scale throughout the game at key points. The impact of +1 bonuses usually overshadows the accompanying IZ unlocks in most cases. By Industrialization, you're looking at 4.5 prod per mined hill on average. The limit on this is how much food you bring in and how many hills you have. Why does this matter? Because even early game, your city can almost exclusively work mines!
Gov't bonus. Assuming you run the legacy card, two governments in particular are very strong for raw production- Democracy and Communism. Democracy's +2 prod per district in a big enough city- say 20 pop, would be about 12-14 prod. Not bad. Communism needs the guvna but would provide 24 production in the same city. Beefy!
Magnus' ability can provide a similar scale benefit to communism, or ~2x more if you actually build the power plants.
So where's the hole in late game? Well, as people have mentioned in past threads on the issue:
Population grows semi-linearly throughout the game.
IZs are one per city.
t3 gov'ts scale with pop.
Mine output scales with population (number of mines worked) and tech (upgrades.) into the industrial era. With their later buildings, IZs can provide an additional 2ish mines per city if you have coverage. By themselves they usually are worth maybe 1 mine, sometimes 2 unless you play germany, japan, or the netherlands (and run the +100% adj card.) Okay, so, while costs are growing linearly all the time, once you hit late game (industrial onwards) you suddenly stop getting qualitative improvements to your industry and are stuck with ones based on population.
You're literally chained back to an agrarian economy because the only way to keep up is to exploit replaceable parts and shove bread into your citizens until they are bursting with octuplets.
Most cities don't have this terrain, but even if half of workable tiles were mines, you'd be at about 2.3 prod/citizen. (One citizen works a farm, the other works a mine.) Then you'd add in the 0.66 prod/citizen from democracy (2 prod per district, which costs 3 pop) or 1.2 per citizen from communism (only applicable in governor cities, but you don't need that many production hubs.) Now, if you put that together, best case we are getting about 3.5 production per citizen as long as we have mines, only 1.2 once we run out. Here's what it looks like, including the maximized work ethic belief:
View attachment 502845
Basically, there's a really big offset right away: there's IZ coverage, the IZ itself, maybe a trade route. Then the city grows and you work those wonderful hill tiles. But at some point, pop 10 in this graph, you run out of mines. Now you can only scale on raw population. Not good. Work ethic is the only main way around this, since it scales infinitely. Nothing like getting +30% production and +36 production from population in megacities of the glorious people's theocracy. What we are complaining about is that we pass that kink in the graph around the industrial modern, but cost growth never stops. Pre r&f, there was no growth in production after that kink! You can raise the offset (build an encampment, a shipyard, more trade routes) but the scaling is fixed.
But hey, if you really focus on production, you can have tons of cities churning out over 100 production per turn with the right civs. The core cities can be spitting out 200+. Where's the issue? Well, the issue is that playing the game to the limit means chops and campuses, not IZs and mines. (And stacking colonial taxes, Amundsen scott, Casa del Cont, amenity boost, dark age cards, etc...)
Solutions: if people think this is a problem, then we have a few levers to pull. The first is arithmetic vs geometric growth phases of the game. In the early game, your cities are growing and you are increasing the number of cities. Geometric. After a point, you usually stop founding new cities, so you have just linear growth of population. Arithmetic. One could divine ways to shift that tipping point later. This doesn't impact the number of turns it takes to build something, just how much you can do in a period of time.
The next thing is change the cost scaling to match production. I won't go into this but there's probably an optimal number of turns a late game item should take to build, perhaps it could be rebalanced around that.
The last thing is to change how production scales. For a lot of personal reasons, I'd lean towards this capacity coming from districts and policies, not inherent boosts; it wouldn't be very exciting if we adjusted per-citizen yields to include 0.5 production in addition to 0.5 science and culture. Even expanding the veterancy card to other districts would be helpful.
I think the legacy bonuses of democracy and communism should be mechanics that come in via districts and policies, because they make the two governments lack a 'je ne sais quoi' intangibility to their flavors.
Example: factories give a home city bonus of prod per pop, power plants give a home city bonus of prod per district. Communism gets some terrain oriented bonus (mines & quarries produce more hammers) and democracy gets something district oriented (IDK. Conditional adjacency boosts? get off my back.)
TL;DR the current meta is not conducive to production times, game balance isn't conducive to production times. I love hearing how people struggle with and get around this differently!