I'm on Windows 10 v1909 i5 4th gen Haswell, Nvidia GT 750M 1GB. I've also noticed the game slows down at the later stages. I had another DX9 era steam game that crashed in windows 10 but runs fine in a vmware virtual machine running windows 10. So I tried running CIV III in my virtual machine and it runs speedily without any delays on the same hardware! I can only guess the emulated processor and emulated DX10 graphics card are more compatible. Not an ideal solution I know but at least something you can try with trial versions of vmware.

Necro commenting, but did anyone else solve this?

The game was written in the era of single core processors. Modern processors unfortunately are not optimized for the way the game is written.

There are couple of things you can do to speed it up by turning off most animations. IE, with a map/game loaded, open the preferences, turn off any checkboxes that say 'Animate'. You can further improve it a little by also turning off any of the Show X Moves options.

For late game the main slowdown I've noticed is it is incredibly bad at calculating trade routes - so every time you finish building a road on your border than connects to another civ's road network it causes a massive slowdown while it re-calculates the trade routes. It also triggers any time you or someone else builds a new harbor or a new airport.

Airports are the worst. Whenever all the civs reach airports at around the same turn, the next couple of turns are brutally slow. I wish it would just resolve whatever civ finishes building on turn start and then calculate trade routes a single time, but nope, it does it anytime anything is built that affects trade routes.

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It's also possible it's entirely Microsoft's / Window's fault and there is no fixing it: this app I'm a developer on, we had a feature that without changing a single line of code, the app on Win XP / Vista was fine - functioned at live speed, but on Windows 7 (and forwared) the same feature took seconds. We never did get Microsoft to fix whatever they slowed down - we had to rewrite the feature in a completely new programming language and toolkit to get it back to live speed.

Windows still support a bunch of legacy API calls but over time some of those APIs are found to have 'security' bugs that get patched. Those security patches for any of the legacy APIs rarely break backwards compatibility, but often cause the response time to be horrible.

I suspect that's what happened with my app was Windows 7 something we were tapping into at the Windows API level was security patched that then causes the same function call that returned in milliseconds to return in seconds.

I also suspect that's a likely reason certain things like the trade network calculations are painfully slow. Microsoft is not going to fix what they broke because it's legacy. The only fix (which is unlikely to happen) is for this game to be re-written on more modern frameworks and utilize multi-thread computing for the more intense calculations.

Use DDRAWCOMPAT wrapper - you can also fix some things such as font aliasing (purple text) with it - check out the config file options. The frame rate will be capped to your monitor's refresh rate (vsync is forced) - but your mouse won't be limited by it (no input lag) - the frame rate will be maxed out when the screen is moving (during turns, moving your mouse, opening menus) but it will idle at a lower rate when just sitting there (normal game behavior as far as I know) It also restores the steam overlay which is a plus.

Thinking outside the box, I don't know much about Virtual Machines, but thinking if it has to be played in a certain way like single core or something, and greatly less specs in other aspects.

I'm pretty sure a modern machine can create some virtual machine that is picture perfect in what is needed to play this game.

So, I feel like its only a matter of time before someone makes a Docker Desktop tutorial, play 2004 games easily and smoothly just do this VM setup, boot a game within it. Smooth

The other option is, they remake and its compatible with todays technology or they just tweak it and make it compatible with todays technology.
Figure it being on steam they would make a friendly today version of the game, but thats not the case it seems.