It's not just you. Gmail has become so slow to load for me that I wonder if they've deliberately sabotaged it on Firefox. Amazing how it's 2019 and web apps somehow seem to be less usable than they were ten years ago. I miss fast desktop apps.
If a chunk of the time, effort and money that went into making web UI so nice and easy to work in could be invested into making true desktop apps equally as nice (no, electron doesn’t count) then I think we’d be in a great place.