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.