This is a C Quadtree that was written to be faster than "multi grids" and then even single layered grids. It focuses on achieving the highest speeds, only then optimizing memory. Unlike probably most out there, it has actual highly optimized collision detection capabilities, not just support for insert and query.
On my laptop at least, this Quadtree handles 400,000 entities of wildly varying sizes at around 17.5mspt, with 1,000x 1920x1080 queries taking around 0.26mspt. Here's a fraction of the simulation:
If you want to run the simulation yourself, just execute make and see test.c for details. Naturally, the time figures given are only for demonstrative purposes only, as they can vary.
For similar settings to my HSHG (500k same sized entities and other parameters), it yields around 15.2mspt, beating it by ~15%. Finally I'm able to say hierarchical structures rock the game when written properly.
Made for my private use. Stability or correctness is not guaranteed (fork it or contribute), although I have tested it a bunch.