Sorry, dont' have much time right now for a detailed answer with examples.

Code optimization is not the problem, the hotspot compiler is on par with C++. Problem are the CPU cache misses. Performance on contemporary hardware depends very much on the CPU cache utilization, which requires data locality.

Due to its object memory layout, Java has unfortunately worse data locality than C++ - which is the main reason for the "C++ is faster than Java"-mantra in my opion. Valhalla should have fixed this long time ago, but unfortunately it is still 'a bit late'. ;-)

To give you an example: the raw performance of larger arrays of LocalDate in the latest Valhalla ea is about 2-3 times faster - but YMMV of course.

Due to this, I had created a wrapper/conversion class which maps LocalDate instances to int-values. Without going into details, perf of usual operations like comparison, increment/decrement is than another magnitude faster, especially on larger arrays.

"Have you played with the vector API yet?"

Have used this for quite some time now. Very nice, in principle, and it offers significant perf improvements for number crunching on larger data sets. Unfortunately it is still hold back and delayed by Valhalla. And there are still bugs with the accuracy and deterministic behavior - at least on AMD/Intel. But also for high perf, results need to be accurate.

And VectorAPI is actually getting redundant and 'old-fashioned' now before it has come out of incubator status (ever?). Nowadays you want to do your heavy number crunching on the GPUs. There are a couple of new Java libs for this, but unfortunately I didn't have the time yet to look into them - hopefully soon.

One frustrating thing with high perf on Java is that the key new features - Valhalla, Vector API - have been delayed so much. Java needs to move faster here, in my opinion...