Hi there all! I recently ran into an issue where builder during the end game (with many heavy mods) caused CRAZY amounts of lag that slows the late game to a halt. I searched on workshop and the discussions here, with no avail. I did find an interesting reddit thread with a solution that helped, so I wanted to share it here with everyone!
This fix requires 1 specific software (I used Microsoft Visual Studio for this),. You want a software that can open up "code", edit code, and search within the code.

Step 1) Navigate to this folder on your storage drive that has CIV VI downloaded(should be located at "\SteamLibrary\steamapps\common\Sid Meier's Civilization VI\Base\Assets\UI" (If installed on an separate drive from your operating system) or "C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization VI\Base\Assets\UI" (if installed on the same drive as your operating system).

Step 2) Find the file labeled "UnitPanel.lua". Right click on this and start with "visual studios". While this may look daunting to some, we will use a function within visual studio (or other software) to identify a specific line of code that we want to change. Use the search function within the software (if using visual studio, click ctlr+f to make the search function appear. Most other software will have a similar function when using ctrl+f).

Step 3) Search within the file for "bCanStart, tResults = UnitManager.CanStartOperation(pUnit, actionHash, nil, tParameters". Only copy and paste what is within the quotations, not the quotations themselves. I found this at line 572, followed by the value "true". The entire line of code should read as...
bCanStart, tResults = UnitManager.CanStartOperation(pUnit, actionHash, nil, tParameters, true);

Step 4) Change the "true" value to false. The new code should read "bCanStart, tResults = UnitManager.CanStartOperation(pUnit, actionHash, nil, tParameters, false);"

Step 5) Save this document AS THE SAME NAME AS WHAT IT WAS BEFORE. Failure to do this correctly will have the fix either not work or the game to be confused and not have the correct information when calling for it. The named file should not change and should look like UnitPanel.lua. If asked to overwrite a previously named filed, press yes and overwrite it. You are just installed the "patch."

Step 6) Enjoy the late games of modded Civ!

Please feel free to reach out with any questions or concerns as I will be happy to troubleshoot and help. If you mess something up, uninstall CIV and reinstall it. That will fix whatever changes you made within the file.

Godspeed