After messing around with the unpacked files for the beta I decided to mod out the mines just to see if it would speed up the gameplay or if some other factor was causing the lag and desynchs. (all i did was alter the cost of researching the requisite techs to 9999999 credits ensuring they'd never be used.)
After this I played a long (8 hours and going) game over lan with my lover on on a large map (aerolian sector) and didn;t have a single desynch, crash or even noticeable slow down.
This was a huge improvement over the previous issues with desynchs after <2-3 hours and the game slowing to a crawl after 5-6 +
I remember Swords of The Stars tried to include mines in their game and it caused no end of headaches game speed wise (and a fair number of desynchs for us due to the sheer number of them) and this seems to be a similar case here.
Like others have said, limiting the amount of mines one can deploy (and makie the fewer ones mooe powerful) and/or put a duration limit on them (around 10 mintues maybe?) could solve a lot fo the issues.
Or leave it as it is and throw in an option to turn them off if you want (or turn them off automatically on med/large maps?)
Still it was nice to enjoy such a long game without any crashes or issues, mines aside, the rest of it's plenty fun. We're going back at it tomorrow with the same game so I'll post back if there's any desynchs or whatever.