For what it's worth when I tried MineTest [1] and Minecraft on my oldish laptop (ThinkPad X61s) MineTest performed significantly better. Determining how much of it is due to the choice of programming language would require a more in-depth performance analysis.
A clones like this have the big advantage of knowing what the end result should look like in advance. That can aid design decisions regardless of language.
[1] A FOSS Minecraft clone written in C++, http://minetest.net/.