Because it's not problem with texture quality, but amount of triangles in view, and internal representation of those. If you have 4GB of VRAM, lowering textures quality will gain you probably just 1-5% of GPU performance, and will do exactly zero to elevate CPU burden.
"There seems to be no fast fix to lower CPU usage or make use of more CPU threads"
Exactly. No easy fix. Except remodelling the Westhill in some low poly mode, like SEGA arcades from ~1995...
Or reprogramming whole LFS engine to be multi-thread. Actually remodelling content in low poly would be easier task, than rewriting the engine (just to give you idea, what kind of task is ahead of Scawen, because unless he drops LFS development, one day he will have to do that anyway, probably much later, than sooner). But then again, people already moan the LFS doesn't look up to date enough, so the low poly version wouldn't satisfy them?