Mercurial is too slow. Even with chg common commands like `hg status` may take hundreds of milliseconds. JJ is fast. Internally `jj log` taking more than 500ms is tracked as bugs.
I am not sure that I understand you correctly. Are you really claiming that lack of speed is why Mercurial did not get traction, and an argument in favor of Jujutsu/jj getting traction?
I am not sure that I understand you correctly. Are you really claiming that lack of speed is why Mercurial did not get traction, and an argument in favor of Jujutsu/jj getting traction?