Many years ago I was using fossil for OpenBSD development to manage my patches.
Around that time I tried to import the entire OpenBSD src repository into fossil, by importing the CVS-to-git conversion of src, as published on Github. I was following the official git->fossil migration guide. I left this running for a week (or two?) at which point the fossil git loader was loading OpenBSD commits from somewhere around the 2000s. At that point I stopped the process. Performance might be better today, I don't know. And perhaps post-conversion run-time performance is much better, but I never got that far. Anyone can try to reproduce these results by running the same conversion today.
I don't think I ever talked about my attempts with fossil to anyone at the time. But I recall the topic coming up somewhere when the Game of Trees project became public, and someone suggested I should be using fossil instead.
I am now using Game of Trees for all my OpenBSD development work and I am happy with it.
TLDR imported cvs2fossil, then fossil to git and mercurial. fossil was less optimized at scale. No dvcs was perfect. At scale things don't always import easily
"The number of artificial commits and conversion glitches could be
minimized by cleaning up various issues. This was made easy by exploiting the
database and writing small Python scripts calling “rcs” and related programs.
Various tests show that the Fossil repository and CVS give exactly the same
output for individual revisions. Differences when comparing working copies of
specific branches are accounted for.
The performance of Fossil is competitive. Some areas like “fossil pull” need
further work, but the majority of the work can shift to improving the user
interface."
Was able to find any and would be curious to read more.