I am playing with a friend since some days, and i can not confirm any better performance.
I hosted on my Xbox one S and my friend joined via Xbox One it worked for quite a while but after 10 hours of playtime it got worse and worse either my system is crashing or the fps is getting to low to play. (all on a new save game btw) Besides that there are still many bugs like backpacks disconnecting from a joined player desync of the formed terrain most of the time stuff can't be done by joined players or they die just walking over something that was build by the host.
Then i hosted a game on PC and my buddy joined on his xbox and things just got worse for him while my performance was way better without him in the session. For example i started a new game on PC alone played for about 2 hours with 140-120 fps but as soon my friend joined it dropped down to 80-70.
Most of the time he would die from jumping or walking over things, getting stuck in the planet, getting kicked out of the game, losing stuff from his backpack and getting disconnected while something of this happens.
I got this game as it lunched as an Alpha and to be honest i don't see a huge improvement performance-wise since then not for the multiplayer part.
I really hope this will be getting better in the future because all the new features won't help if you cant play the game to the end. But i am also exited to test the dedicated servers i hope i can run them on my Server PC without a subscription.