Have you tried comparing it with Bromite, Brave or even Chrome? There is a very noticeable difference.
Also, I think Site Isolation* doesn't work in FF Android as it does in FF Desktop. Firefox Android is categorically less secure than its Chromium counterparts.
I have not done much comparison, as it works fine for me. I could believe that it is slower, but I still wouldn't call in slow. Perhaps my using an adblocker with FF makes it better for me.
Also, I think Site Isolation* doesn't work in FF Android as it does in FF Desktop. Firefox Android is categorically less secure than its Chromium counterparts.
* https://hacks.mozilla.org/2021/05/introducing-firefox-new-si...