Interesting, I am also suffering from 3-5 fps using a 3900xt, 64gb ddr4 (2800mhz) and a 2080.
My concerns are that I play in 4k, also have massive ships. (I use around 4x 29 SS-17's to 2 stage to Jool and outerplanets in the outer-planet mod.)
Haters may say that wow you really dont need that many solid boosters but i'm part of the Robert Traux school of thought when it comes to spaceflight.
My fps varies from 1 fps during launch, 7fps in orbit, 17 fps using a drop ship to other planets. When redocking to main ship or in close proximity it drops back down to 7fps.
Have you tried using task manager to give the KSP.exe high priority? (It didn't do anything for me, but may in your case)
What has worked somewhat was increasing the ram speed. Using 2800mhz instead of a lowerspeed increased fps by 1-2. Furthermore, using Precision X-1 and overclocking my gpu gave me simular 1-2 fps gains.
Now though, i feel that this is unacceptable and really would like to get to the bottom of this.