It wasn't the Mercurial team saying it was faster than Git; that was Facebook after contributing a bunch of patches after testing Mercurial on their very large mono-repo in 2014 [1]:
For our repository, enabling Watchman integration has made Mercurial’s status command more than 5x faster than Git’s status command. Other commands that look for changed files–like diff, update, and commit—also became faster.
In fact they liked Mercurial so much they essentially cloned it to create their own dvcs, Sapling [2]. (An aside: Facebook did all of this because it was taking too long getting new engineers up to speed with Git. Shocker.)
Today, most of the core of Mercurial has been rewritten in Rust; when Facebook did their testing, Mercurial was nearly 100% Python. That's where the "Mercurial is slow" thing came from; launching a large Python 2.x app took a while back in the day.
I was messing with an old Mercurial repo recently… it was like a breath of fresh air. If I can push to GitHub using Mercurial… sign me up.
[1]: https://engineering.fb.com/2014/01/07/core-infra/scaling-mer...
