Note: Mekanism Community Edition doesn't have a fix for this.
I had implemented 2 mixins that fix block culling in mekanism generators some time ago. I don't really know if it impacts rendering performance much, but it should be noticeable when at least ~3 big mekanism multiblocks are built.
The issue in a nutshell: the blocks used in multiblocks always have all 6 sides rendered, even if there is a block touching a side. Any minecraft block by default does obey culling and does remove a face adjacent to another non-opaque block, but mekanism devs didn't bother and they set every generator and reactor block to be opaque (aka "transparent")
Mixins: BlockGenerator and BlockReactor