When I meet laggy in Minetest, I can usually see this
In face, players can't see the underground world, so on the server side, there's no need to send those block data to the client if the block's depth is too big, and on the client side, don't make block-meshes for those blocks, they're invisible.
I don't know the details of the Minetest engine, Minecraft is fast on my android phone, but Minetest is laggy. we all want no laggy Minetest :)
Does anyone here know how to fix this or is this will be fixed in the futures?