In heavily performance-engineered code having a GC coalesce memory is a pessimization in all cases.

I've done a lot of performance engineering in both C++ and Java. Every optimization available in Java also exists in C++ but the reverse is not true, which is why C++ is always faster. Every example I have ever seen of Java being faster than C++ was just poorly optimized code. The heuristic I use is that heavily optimized C++ is about twice as fast as heavily optimized Java at the limit. And this requires some non-idiomatic and ugly Java that isn't nice to maintain.

This doesn't necessarily make Java the wrong choice though. Few organizations prioritize absolute performance above all other things. There are practical tradeoffs.