It wasn't a little speed. Other options were many times slower. I just renamed a large subdir in a large project. `time git status` took 41ms. That kind of speed lets you add all sorts of interactivity that would be impractical if it were slower. For instance, my shell prompt shows whether the current directory is managed by Git, and if so, whether the status is clean. I would never tolerate my terminal being detectably slowed by such a thing. With git, it's not.

There are a thousand little ways where having tooling be fast enough is make-or-break: if it's not, people don't use it. Git is fast enough for all the most common operations. Other options were not.