We've tested this a lot and it seems a performance issue of either the GPU, CPU or both.
I have to disagree.
With civ5 i had this issue on a 980 with a friend who had a 760, both running high end i5s for the time
now i we have a 1080 and a 1070 both running high endi7s and we randomly will get desyncs. We have now played 2 full games without desyncs just by going to full screen.
So it may be the GPU, but not because they are low end. We have both always had flag ship top of the line GPUs when having desyn issues.
Actually even STILL playing civ4 and civ5 we get desync issues with our 1080s and 1070s...
edit:
and size of map doesn't seem to factor in one bit for us. We play on tiny - ultra huge (the modded map sizes) and the desyncwere just as random.
Also in 99% of our games, there are no mods, so not a modding issue.
And if anyone says it must be your internet, that is not true either. I have desync on LAN with 2 computers 5 feet apart on a nice little switch that gets desync.
edit2:
I have yet to try and turn down the gfx on all players. I have always just kept mine as low as possible since i am not a fan of gfx. I actually play in strategic view most of the time too
I would be ashamed such a persistent and repeatable bug for 2 years and not fixing it.
This has been a bug since civ3 PTW. Although back then the game would actually fully desync and NOT tell anyone. you would see the other humans acting all ******ed and their starting city would be like in the middle of the ocean....