Honestly, this is only because `git checkout` is so convoluted that we've collectively changed our expectations around the UX. "checkout" can mean switching to another branch (and creating it if you specify a flag but erroring if you don't), looking at a commit (in which case you have "detached HEAD" and can't actually make changes until you make a branch) or resetting a file to the current state of HEAD (and mercy on your soul if you happen to name a branch the same as one of your files). Instead of having potentially wildly different behavior based on the "type" of the thing you pass to it, `jj edit` only accepts one type: the commit you want to edit. A branch (or "bookmark", as jj seems to call it now) is another way of specifying the commit you want to edit, but it's still saying "edit the commit" and not "edit the bookmark". Unfortunately, the expectation for a lot of people seems to be that "edit" should have the same convoluted behavior as git, and I'm not sure how to bridge that gap without giving up part of what makes jj nice in the first place.

I doubt it wouldn't be significantly slower. I can't disprove it's possible to do this but it's totally possible for you to prove your claim, so I'd argue that the ball is in your court.