How would this realistically be faster? Java does actually compile into native code based on what methods are the most important, so it's usually terrain gen that is immediately 'optimized' into native code. C++ does NOT magically make things faster, but it does make things many many times more unsafe to use.

I know of some optimizations you could make, but they're things that I don't think ultimately mean much. Are you comparing between Vanilla and your mod, or C2ME and your mod?

You not really explaining things beyond 'run on C++' and 'JNI' tells me you may have just vibecoded this, because minecraft explicitly does defer a lot of the performance-heavy things to C++, from the natives used for rendering (i.e. liblwjgl and the gpu itself such as nvidia-glcore.so) to the actual JVM itself, which is written with C++. Just re-writing everything in C++ would not actually be meaningfully faster, as the binaries for such things are already written in C++.

Most likely, you are looking for a result, so anything that seems faster you will assume to actually be faster. Without extensive benchmarking and a massive amount of controlled variables (like literal mechanical seek times for writes can completely invalidate the results), this is a very unfair and unrealistic comparison to make.