A big part of my project Flux Empyrean has always been the dynamic sky, but it wasn't until recently I realized how big of a performance impact it had.
The game ran fine, always above 70fps but it was never very consistent. The sky at the time was just a bunch of individual meshes that formed the stars.
Out of curiosity I looked into MultiMeshInstances. I had used them before in previous games, but I didn't realize that you can individually set mesh positions and shader parameters. Using that knowledge, I was able to pack the sky into a single MultiMeshInstance3D and it ran better than ever before, with 100x more stars than before!
Hopefully this helps someone! I wish I knew that you actually had a lot of control in how things get placed with them, I went so long assuming they were purely for random foliage style effects.