Here are some numbers from my 7 or 8 year old system:
Ryzen 7 1800x, 16GB, nVidia 1080
With the display mode set to full screen, the numbers were OK-ish. A little better than I expected. Then I changed to Windowed mode, and the numbers went up a little and were even more OK-ish.
Space Center Menu = steady 30fps
VAB = Steady 60fps
Launch Pad = 25-30ish fps
Launch (stock K1 rocket) = 19-30ish fps (increased at higher altitude)
Orbit = steady 60fps Looking out at space. 20-ish if you are actually looking at something, like Kerbin.