[Steam][][Coop] Multiplayer issues

Hey guys, I have been playing a coop game with a friend of mine lately. We made 2 simple custom islands in a new world. He is the host and I am the client. We both have a pretty solid internet connection (100MBit/s +)
We are experiencing desync issues, the client (me) has troubles with dropping stuff sometimes, minor lag (More lag if we play on really high detailed maps), and some really weird hard lag if we try to move our self made raft.
The raft is 4x3, has 12? shelfs for boxes, 1 sail, 1 anchor and a motor. If the host tries to pull it, the client (me) experiences really hard lag and if he does not stop pulling it, a timeout. We build a smaller raft, 2x2 with a motor, anchor and 2 shelfs and no lag there.

This would not be a huge problem, if we did not have to load for 5+ mins when reloading on a timeout, desync, bad lag problem.

I am playing on a wide screen monitor, 3840x1600 on medium settings. As I said, we added two island to the otherwise virgin world.

I tried to activate FPS limit to counter the lag problem, I tried on every possible resolution and game quality, window mode and full screen mode. No differences there.

If there is anything else I can provide, please let me know.