It was very noticeably slower on all but the smallest projects. I started with it but switched to Git later for some open source work and that was one of the first impressions I had.
According to Facebook Mercurial was faster for them, but that was in 2014 and they had to put work in to actually make it fast. Their use case is rather outside the mainstream of course.
"When we first started working on Mercurial, we found that it was slower than Git in several notable areas. To narrow this performance gap, we’ve contributed over 500 patches to Mercurial over the last year and a half."
Personally I can't recall any serious performance difference after I switched from mercurial (which included some large-ish repos) to git, but it's been quite a few years ago and perhaps I just forgot.