The mod already implements culling for item frames, an entity which cannot have faces culled in vanilla. Paintings, another entity, are a prime candidate for something to do next: by design, not only their back faces, but every face that isn't their front will always be flush against a block face, making them impossible to view if there happens to be a block occluding that face. So having these faces be culled should probably be a performance boost as well.

Also, while I haven't tested this with the mod, in vanilla paintings appear to have completely redundant internal faces when larger than 1x1, so having these faces also be removed would likely be beneficial since those can never be seen at all without clipping inside of the painting.
Likewise, it'd probably be a good idea for the edges of paintings to cull the edges of other paintings, if that's even possible...