Git is incredibly fast. Many operations happen almost instantaneously even on large repositories. It's really nice. I've heard a lot of people talk about how they abandoned mercurial because it's slow. Some even switched to git from svn not because git's model is better but because svn is slower.
"Premature optimization is the root of all evil" is an old programming advice quote that is so horribly misunderstood that it's basically wrong. It means think about your problem at the high (algorithm and overall goal / design space) level before worrying about low-level optimizations. It does not mean that optimization is bad or that performance isn't important.
I made another comment recently on the same issue. I really wish that quote would die in favor of more substantial advice that won't be so misunderstood.