Maybe this is a sign that monorepos are the problem?

Why would this indicate that monorepos are the problem?

It’s a philosophical conflict. Git was designed for distributed, decentralized version control and collaboration. Monorepos are inherently centralized. Thus git is the wrong tool for the job and all of this effort to adapt git to monorepo workflows runs counter to its philosophy.

I think the monorepo workflows which Git isn't good at are things like branching, code review, and feature/version management. But there's no reason that Git should have to be slow just because a repository is large. It's more things like "merge commits don't scale in a monorepo", which I would agree with, but that's not related to the performance of the index data structure.