I've had a lot of fun in KSP 2 since the For Science! update was released. However the issue which is covered in this bug report has started to be a significant issue for me.
https://forum.kerbalspaceprogram.com/topic/219210-ksp2-is-calculating-the-physics-of-all-parts-of-all-crafts-whether-they-are-rendered-or-not-reducing-performance-of-all-scenes-at-all-times-updated-to-0140/
In my savegame I have 25 vessels total of which 5 is small satellites with few parts. The rest are mostly interplanetary vessels with typically 20 of the medium hydrogen tanks, 10 nuclear engines and various other parts. My way of playing the game might be a bit unique, but not an unreasonable number of vessels or parts in the big picture.
Last night I had an idea to start a separate empty save to design and test vessels to get a break from the poor performance I have played with for a bit now. When I went back to progress on my main save it really hit me how bad it had gotten. The game does 12 fps on the main KSC screen and 7-8 fps when I launched my latest vessel. In my empty save I had 20 fps while launching the same vessel which felt fine. I'm considering taking a break until some significant improvement has been done on this issue.
Wanted to make this post to hopefully bring it higher up on the priority list in case more people are experiencing the same thing. There are many things to work on, but for me it would be a game changer to fix this issue. The game is significantly better now in terms of bugs then at the beginning of early access. The bugs can be annoying and a bit time consuming at times, but they can usually be worked around. This issue is constant and the only workaround is doing a 180 on my plans and make fewer and smaller vessels.
Here are my computer specs for context:
Ryzen 5800X
RTX 3070
32GB RAM