Brushing the dust off this account to say I have noticed an issue during my playthrough, Loaded Kopernicus by it's self and still got the same issue.
It seems that when I approach the ground my GPU drops to an idle state. My core clock goes from my usual 2000 mhz clock to it's idle clock of 1600 mhz as well as my GPU load drops to 10-20%.
https://imgur.com/a/b4CqAaY - Screenshot of my perf overlay. You can see the low GPU clock and usage, the low power draw (Right of clock) and the limitation reason "No Load". You can also see my CPU is not a bottleneck and I have 32gb of ram so that is not the issue.
https://pastebin.com/c4unYGRC - This is the log from KSP, Though I went through and nothing jumped out at me.
I was going to do more research with ground scatter and textures etc. But I don't have that kind of time sorry.