Did you have a warm cache for either? I find that Fastmail’s cold load is generally a fair bit faster than Gmail’s cold load, and Fastmail’s warm load is consistently faster than Gmail’s warm load; but Fastmail’s cold load can be slower than Gmail’s warm load.
I’m looking forward to working on service worker code loading loading and offline support in Fastmail; I expect us to get warm loading to under one second on a typical desktop machine, regardless of location (because it will require no network calls).
Either way, once the code has loaded, Fastmail is consistently snappier than Gmail. And uses far less memory (in Firefox, 10–15MB instead of >150MB). Offline mode will improve snappiness even further even if you don’t opt into actual offline sync, by virtue of persisting its data record cache. But that’s not implemented yet.