Alright, I think i've figured it out.  Either that or it fixed itself at the most coincidental time ever.  

I'll preface that both PCs are Windows 10, fully updated including drivers, on the same network (500MB/50MB hard wired, not wifi), Steam as the platform.

Before we start the game, every time, we open Xbox Networking and "check" the connection which usually requires the "fix" button afterwards.

 

Just like everyone else, we had issues where the "joiner" would begin to lag and/or get disconnected multiple times.  Now for the fix:  This worked for me, hopefully it works for you.  Have the "joiner(s)" turn off auto-save.  Voila, no lag, no disconnects, no stuttering.  Nothing.  We went from being able to play for about 20-30 minutes at a time, to 3+ hours straight without issue.  I don't know if it is an issue with the new shared save feature that causes a conflict when multiple clients are attempting to save to the same save file in the cloud, or what.  But this has seemed to fix the problem completely for us, and the game is once again playable!

This didn't work for us, but thanks for posting a possible solution.