It was performance that made git win initially. In benchmarks git did better in the past when there was still a great Cambrian explosion of DVCSs (Monotone, Bazaar, Hg, Git, Darcs,...)

It is of course possible that performance follows a saddle like curve (or hump depending on how you look at it). For up to some size, git is faster. But at much larger ones a different algorithm wins out and hg could be end up being faster.