I believe librewolf updates very quickly, to be fair, it's really stock firefox with just some privacy/performance tweaks that are probably automatically enabled and then run through CI and then released.
No, the updates are not slow. I've been running Librewolf for years now, no major complaints. FYI I'm on a rolling release (linux) distribution, but I doubt that changes much. If you are comparing it to stock Firefox, there is no competition (Librewolf is imho just more enjoyable experience); if you are comparing it to Chromium (and other Chromium-based browsers), in my experience the biggest 'problem' is the lack of support for pwa's. Anyways, give it a try.
I really don't like Firefox forks, for the slow updates and because I do genuinely use some bleeding edge features, but I'm tired of Mozilla.