> and total energy usage

Surprisingly, Java is right behind manual memory managed languages in terms of energy use, due to its GC being so efficient. It turns out that if your GC can "sprint very fast", you can postpone running it till the last second, and memory drains the same amount no matter what kind of garbage it holds. Also, just "booking" that this region is now garbage without doing any work is also cheaper than calling potentially a chain of destructors or incrementing/decrementing counters.