Thanks for the reply Kallocain. That definitely was the issue for singleplayer performance, however, in multiplayer it's a whole different story. I can see that the multiplayer is definitely much more intensive than singleplayer and results in way lower FPS values, that has been the case for me with the past Paradox titles as well, unfortunately.

Is there anything that can be done to fix the multiplayer code, to make it run smoother? Because at the moment, this is how it is for me from a performance point of view:
Singleplayer > multiplayer when hosting > multiplayer when joining. Singleplayer offers me excellent performance, multiplayer when hosting offers much less performance, and when joining... Well, that's when the FPS is like 15-20 on my system, not really a value that I think is playable (for me at least).
If something could be done to improve the performance in the multiplayer part of the game, I'd really, really appreciate it.
Also, was the windowed mode changed from the preview version to the release version? I prefer the windowed mode of Victoria 2: A House Divided to that of Crusader Kings 2, because when I use my monitor's native resolution in windowed mode, I can then use a third-party program (ShiftWindow) to maximize the whole window, and simulate windowed fullscreen mode. However, that same program does not work with Crusader Kings 2, and the taskbar remains on the bottom. Speaking of which, it would be great if a "windowed fullscreen (no border)" is implemented in the game, that would really help us streamers. Streaming fullscreen applications can cause issues.