Yeah, inbox seems vastly superior to me, with the exception that it runs like absolute shit on Firefox which I can only interpret as intentional, considering I find FF faster than Chrome for a lot of other stuff these days.
It's trivial to explain the speed difference without assuming it intentional. In Inbox's case, sparse arrays were an issue initially[0] (and like many of the perf issues, it's been fixed[1].)
There will always be perf trade-offs. If you find any where they chose X over Y when X and Y are equal on Chrome, and X is faster than Y on Firefox, then let's talk about anti-competitive behavior.
[0] https://news.ycombinator.com/item?id=8495498 [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1087963
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)