> Why does it matter?

Because you need to have an idea of how many machines you need. And you need an idea of whether 'it will take as long as it takes' or if there's an issue in your code.

Sometimes Java decides to JIT using aes instructions. Sometimes it doesn't. Even if you run the same benchmark suite twice in a row it just does its own thing and gives wildly different results.

My learning was that one shouldn't depend on JIT performance and prefer using JNI libraries if you want consistency. Hence my comment being negative on JIT.