I know this is an old thread... apologies!
I think I have further narrowed my performance woes down to Clouds and Scatterer. Scatterer by it self runs decently well. Basically, my mod list in short:
√ Spectra v1.6.5 1.12.5
√ Spectra EVE Minmus geysers 1.6.5 1.12.5
√ Spectra Scatterer v1.6.5 1.12.5
√ Spectra Scatterer for Minmus 1.6.5 1.12.5
√ Spectra Scatterer Jool shine on Laythe 1.6.5 1.12.5
√ Spectra Scatterer vibrant sunsets 1.6.5 1.12.5
Spectra EVE 64k Kerbin clouds 2.0 1.12.3 <-- nope
Spectra EVE Kerbin and Laythe snow 1.6.5 1.12.5 <-- have not tried, but says its laggy
I can mix and match a manual install of Scatterer and other Environment enhancement mods. But no matter what, when I introduce a mod that brings in clouds, I get drastic FPS hits. In closing, the above looks and performs well on an Apple GPU.
A little off topic, but once in high orbit, I found other performance hits related to how many Kerbals were being displayed in the bottom right of the screen. Limiting that, or out-right turning it off increased my FPS. Neat find.