Just noticed this today, but all 64x cubes do not cull. Culling is when unseen faces are hidden, reducing framerates dramatically.
(left to right: default cubes, zone bricks, terrain cubes)
INCORRECT CULLINGCORRECT CULLINGIf you don't believe me, download the save below and try it for yourself. I've tried it on two different servers, and with the freebuild gamemode.