I don't think it's hyperbole to say a significant percentage of Git activity happens on GitHub (and other "foundries") – which are themselves a far cry from efficient.

My ultimate takeaway on the topic is that we're stuck with Git's very counterintuitive porcelain, and only satisfactory plumbing, regardless of performance/efficiency; but if Mercurial had won out, we'd still have its better interface (and IMO workflow), and any performance problems could've been addressed by a rewrite in C (or the Rust one that is so very slowly happening).