That’s something broken with your Firefox install or computer then. It’s very performant on any of my two G Suite and one Gmail accounts on Firefox as well as Chrome.

Or something broken in how Gmail handles a wide variety of Firefox installs/configurations (this is a common reported issue). The fact that you personally haven't seen it ("works on my machine!") doesn't mean the issue doesn't exist.

Interesting. Are you exaggerating a bit here or being accurate with those timelines? Cos Gmail is really slow on Firefox for me too, but it doesn't take half a minute to load. More like 10-15 seconds, which in this day and age is a nonsensical amount of time. Similarly operations have a 2-3 second lag. Noticeable to normal users but people like my parents wouldn't be able to see it.