I think you're probably right. I'd imagine that most of the code in Archive Utility.app is very old, and there's very little business reason to spend any engineering time on improving it beyond doing just enough work to make sure it works with API changes for later OS releases.

There are third party GUI compression utilities that are faster that you'd find if you actually care about how long it takes to decompress your archives, and the CLI utilities are all there out of the box for people who don't need a GUI.