I am a university professor, game developer, and 4X TBS game fanatic. I played, developed, collaborated on, play tested, analyzed, wrote about, and searched for 4X TBS games most of my life.
I played thousands of games and tens of thousands of hours of TBS games. I make this post after 300 hours of playing Old World. My estimated level of knowing and understanding Old World and its features: 60%.
Far from being perfect, but my perception is that Old World is currently the second best game out there. (I may be biased to 4X TBS, although I played many other genres).
Old World has the usual big problems with the UI: cumbersome, confusing, slow, buggy. Its AI seems to be somewhat better than the other 4X TBS AIs. As of today, 4X TBS AIs without exception are very poor based on my personal experience with hundreds of these games.
But Old World’s design is one of the best I have seen so far in any 4X TBS game. Finally, I see Order Points that limit the number of actions per turn and limit my movements. I advocated for this for 30 years in 4X TBS development. This game has great feature diversity, depth, and attention to details. Many things can be customized, even the music. I am yet to try the multiplayer. The developers genuinely understand and care about their product which combo is rare to find in the 4X TBS genre. This is not just another dumbed down “clone” of an earlier game that dominates the 4X TBS market today. They still push new patches which is great. This game is similar to Civilization 4, but much better. It also has tactical depth, role playing, and probabilistic planning, and a lot more. Kudos to the developers and I wish they will provide many years of support to fix the bugs, improve the UI, further tune the balance, and strengthen the AI to make the game even better than now.

Bought this in the run up to Civ 7, but i'm scared Civ won't be able to live up to Old World now. The best and most in depth 4X game on the market.

Starting a new game, exploring the map and setting up your first few cities is fun enough. The troubles start when most of the map is revealed and you have more cities and control a larger area, that's when the game starts having serious performance issues.
Selecting a worker in the late game stutters the game to a halt. Turning off 'show building recommendations' in the options helped a bit, but longer games are quite unplayable as moving a single unit takes several seconds as the game hangs and the units do not move. This happens with nearly every menu item or a unit in the late game.
I tried playing a medium sized map with a lower amount of cities, but it still happens - I'll defeat two opponents and as the area I control gets larger and the game gets gradually slower and slower, while testing my patience.
Why does the game have to render previews for items on the other side of the map when selecting a worker?
Selecting a larger city gives you an abundant amount of information on the screen when I can just check the total value at the top of the screen and the available upgrades on the sidebar on the left - all the useless numbers that the game loads and shows in the hundreds of small boxes on the screen slow down game to halt - I wish there was an option to turn them off, it's all just bloat.