Codon slower than PyPy, can't find out why

Hi, love the project!

I recently started implementing a ray tracer as an exercise for trying out Codon. After a while, I was curious to try and make the code work with vanilla Python and PyPy, and then found out that my renders are about twice as fast with PyPy compared to Codon.

After trying a few optimizations to no avail, I decided to try and profile the execution of the Codon-made binary:

It appears that more than half the time is spent on some internal `gc.alloc_atomic`, and also starting threads? (I have zero `@par` in the whole codebase).

I noticed that when using `time`, the `user` time is often twice as much as the `real` time (something in a thread is doing something). And in turn the `real` time is still twice as much as PyPy's.

My suspicion is that creating a lot of `Vec3` classes all the time is somehow bogging down the GC. Maybe I have a basic misconception of how to use Codon?

[Here](https://github.com/exaloop/codon/files/15129358/rtow_codon.svg.zip) is an interactive version of the flame graph (unzip and open the SVG in a browser), and the code is available here: https://github.com/Tenchi2xh/RTOW-Codon (check out commit `379d5d0`, the master branch now has other types of optimizations). The main entry point is `rtow/__main__.py` but it's easier to run it from the `run.sh` script (a preprocessor has to remove python-specific stuff). To run it faster, just reduce `samples_per_pixel` and `max_depth` on lines 52-53 (it runs even slower in the profiler)

(Sorry to link to a whole repo, it's not a big codebase, but big enough to make it hard to produce a minimal reproducible example for a Github issue)

I am using the latest dev build of Codon, downloaded from a CI build