Well, speed issues are currently not that noticeable (a lot of times passed since initial bumps) but I'm pretty sure some large repositories (more than millions of lines of patches) wouldn't be able to work with it.

git is a whole different story compared to darcs, darcs is simpler to use, especially when merging.

if it was consistently fast I'm pretty sure git wouldn't be as popular as it is.

there is rust vcs https://pijul.org/ that seems to be doing the same thing as darcs but faster.

so I guess CJefferson's controversial comment stands :D