On the largest repos I've worked with `git blame` takes a second or two: long enough to be annoying and make me wish it was a bit faster, but still fast enough for interactive use and well short of the threshold where I would be tempted to context switch after invoking the command. On most repos it's perceptively instant.

I suspect on a HDD `git blame` would be unbearably slow on anything but the smallest of repositories, but it has been many years since I last worked with source code stored on a HDD.