> Help me here, why do GC improvements cause CPU increase?

In Java 8 (afaik) there were pretty much no generational or concurrent garbage collectors, so garbage collector would happen in a stop-the-world manner: all work gets put on a halt, garbage collection happens, then the work can resume.

If you have a better GC, you have shorter and less frequent needs to do a stop the world pause.

Hence the code can run on cpu for more time, getting you higher cpu usage.

Higher cpu usage is often actually good in situations like this: it means you're getting more work done with the same cpu/memory configuration.