The issues with Minecraft aren't due to the language it's written in, they're down to how certain things in the engine work.
It tries to render entire chunks, even if they aren't visible. It calculates AI for mobs you can never see, etc. And chunk loading is pretty slow too. You could re-write Minecraft in C or such and it'd be just as slow unless you fixed those issues, and if you fixed them it'd speed up the Java version as well anyway.
Edit: And people are mixing up languages and runtimes. Android uses Java, but it doesn't use the Java VM. Any speed increases from porting it to C# come from the different runtimes, not the language.