There was a noticeable increase in GMail loading time (when I switched to FF a while back), but once it actually loaded it was the same speed (slow af on both lol). As an aside, it's gotten even more bloated in the meantime (for some Google PM's promotion), I just use Basic HTML now. Loads instantly :)

So once the page is loaded, JS performance seems to be about on par, but page loads are faster in Chrome.

To measure I just used dev tools. But there is quite a lot of randomness involved -- sometimes the engine scheduling planets align, and sometimes they are way out of alignment. You'd need thousands of samples or more to get an accurate reading. I just refreshed a few times to get ballpark numbers.