I doubt that speed in a package manager would depend greatly on programming language choice. A package manager downloads the repository index, evaluates your current environment, decides what packages you need and then downloads them. You may get minor speed improvements due to a more performing programming language, but we're talking about milliseconds differences in a process that likely takes several minutes. I wouldn't take that into account when choosing across options. Indeed speed can greatly vary across package managers, but that mainly depends on implementation; as such you may have a package manager implemented in a slower language that is faster than one implemented in a faster language.

If I have to choose a package manager, I wouldn't even consider speed and rather evaluate functionality. I don't know paru, I imagine it allows doing what yay allows doing and as such I'd be satisfied with either of them.