It is a lot faster for me... so far, the most noticeable example is when using the back/forward buttons on Reddit (it used to reload the whole page every time)

It is a lot faster for me... so far, the most noticeable example is when using the back/forward buttons on Reddit (it used to reload the whole page every time) but more than half of the extensions that I use haven't been updated yet to work with Firefox 57. Looks like you can enable legacy extensions, about:config?filter=extensions.legacy.enabled, but it appears to be only for installed extensions (you can't add new ones).