I am running Firefox 56 (Nightly) and it is very fast due to project Quantum. Even faster than Firefox 54 which also got a big boost due to e10s.
https://www.mozilla.org/en-US/firefox/channel/desktop/#night...
If Firefox 54 is still slow for you, I would be interested if multiprocess is enabled (look at about:support). Also, what does about:performance say?
If still slow you might want to do a Refresh: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-a...