It's pretty much rpm-ostree. Nobody bothered to make those workflows performant, so if you need to apply updates separately, it's going to suck. The OSTree download can be fast if you have a fast connection to the Fedora server, but it's not mirrored and there's no mirror network support (so no geographically close downloads). To be fair, bootc has this problem too because container tooling in general can't support mirror networks currently.