If it's really painting causing the most lag, and regarding the fact 1.13 will give us many (from the things i saw in the snapshot over 700 new solid full blocks from noteblock and mushroom block blockstates) we could make "drawer block" which basically would be a regular block with the painting's texture on one side. Just an idea for now, it would require some more investigation on whether it's really the paintings causing the most lag, else it probs is not worth the change.