as far as i'm aware, the chiseling system is a lot more dynamic than just "rendering every single voxel", if you've used any chiseling tool mods, or checked out how it separates certain sections of a block when you'd modify it, you can see that Vintage story divides chiseled blocks into neat chunks depending on its shape. the only way for it to "render every single voxel" would be making it like, a checkered pattern of air and blocks, or something crazy like that.
so complex builds that divide each block into multiple chunks- like custom roof tiles with lots of nubs and crevices, or lots of uniquely chiseled statues, yeah, you're asking your computer to process more surfaces like coolAlias said above.