Try jj out on an open source Git repo -- I think it will both be very easy to pick up, since you're used to Sapling, and quickly make sense.
You know how sl amend and sl fold/squash are two different commands, right? Well, jj amend is actually an alias for jj squash. And that's just the beginning.
Regarding uploading, yeah, you'll likely have to have some heuristics, but Mononoke is generally built for very high throughput. The heuristics might just be around local debouncing and aggressive expiry of uncommitted snapshots in the cloud. But in general, it's worth thinking about the local moment-to-moment UX independently of commit cloud considerations.
