My data point is the Mozilla monorepo. My .git directory is 5.5GB, so half of what you're talking about. jj speed has not been a problem at all. Some thing are sped up a bit by enabling watchman, but I didn't bother for the first 4 months or so that I used it because the speed was already fine and I didn't want to add another point of failure. (And in fact, when I first enabled it, it slowed things down because the checkout is too big and hit inotify limits.)

I don't have very many branches, though. 13 branches, 6838 tags.

Some caveats: jj's version of `blame` (jj file annotate) is painfully slow and I usually run `git blame` instead. (Which is a general thing: if anything is missing or slow, you can always just do it with git.) jj's rebase is insanely fast -- especially if you add in the time to manually resolve conflicts, since a good chunk of the time I don't have to spend any time at all since the conflicts are only in experimental twigs that I may never need to go back to (and if I do, it's fine to wait until I need it).