Translation:In my opinion, we must quickly do something.
In my opinion, it is important that we see that "quickly" modifies "doing", and as such, should follow it, not precede it. In French, it seems the "vite" must follow "falloir", but in English, "we must quickly do something" is unnatural and awkward. It is an imperative phrase: "We must do something! (and) Quickly!" and "Quickly! We must do something". However, making this subservient to "In my opinion", deflates the imperative nature. Nevertheless, in my opinion of an "old" native western american speaker, "We must do something quickly" is the most normal, natural phrasing. IMHO. KenAndresen was much more succinct, d'habitude.