[2023] Multiplayer Slow Transfer And Playing

When you hit PLAY in the Steam client, choose the Nakama MP option in the mini-menu you see there. All players will have to do this, as it uses a different MP service.

If that helps, the affected players have almost certainly some Steam ports blocked

Steam Support :: Required Ports for Steam

support.steampowered.com

Good luck!  

That is right - CK2 ceased development before the move to Nakama.  

So EU4 worked fine for a while then this problem returned? If so, the problem must come from outside the game or the Nakama service.

Can both of you please try playing with any third person? This is to determine which of you two have the problem.  

If you have a VPN, enable that before starting the game. That will bypass any firewalls in your router or ISP.