I specifically installed Firefox 57 so that I could try Inbox there, because as you point out, on Firefox 56 it's very slow (especially the initial load).
On Firefox 57 with that new Quantum CSS, I found Inbox to be amazingly fast (I'd say even faster than Chrome on my yoga pro 2 - except for the right pane of Google Hangouts which takes a while to load still)
On Firefox 57 with that new Quantum CSS, I found Inbox to be amazingly fast (I'd say even faster than Chrome on my yoga pro 2 - except for the right pane of Google Hangouts which takes a while to load still)