Since then both had lots of optimizations put into it to work outside their original intended domains. Facebook did a lot to make Mercurial performance on very large repositories tenable. Microsoft did the same for Git to allow their Windows developers to work with Git, which apparently was initially not able to do common operations at all at acceptable speed.

But those are big company problems and I suspect the caveats, tradeoffs and solutions matter a lot less for the 99 % of other VCS users.