Hematite doesn't implement anything near to the full game, so it wouldn't be a fair comparison. It's mostly just a world visualizer, sort of like Overseer is for Dwarf Fortress ( http://www.bay12forums.com/smf/index.php?topic=63484 ).

And although it would probably make a good jumping-off point for anyone who wanted to develop a Minecraft clone, the original goal of Hematite was only to serve as a proof-of-concept of rendering 3D graphics in Rust.

Are you sure they are caused by GC? I'm asking because GC is an easy "culprit" and when a Java app pauses / jitters, many people immediately blame it on GC without any hard evidence. While in fact pauses / unresponsiveness can be caused by many other reasons, not specific to JVM (e.g. improper thread locking, improper use of dynamic data structures, doing hard work on the GUI thread, etc.). Actually, a well coded game should not use dynamic memory allocation in the game loop, neither in Java nor in C++.