PNG : ~15.1 seconds, 83% accuracy
I also tried dropping in zstandard compression: Zstd (level=3) : ~3.5 seconds, 88% accuracy
Much faster than gzip, at least. And iff I use (x1-x2)*2 instead of x1+x2 to calculate Cx1x2 that pushes zstd up to 93% accuracy.
Namespacing is the solution, and as mentioned in the article some ditros do indeed have namespaced user repos, like Fedora's Copr. The trust model of a flat namespace user repo is completely broken when the maintaining user can change at any moment.