You can load gmail is basic html mode, it might help.
According to https://support.google.com/mail/answer/15049?hl=en, the following link should set it to basic html mode (I haven't tested it) https://mail.google.com/mail/u/0/h/1pq68r75kzvdr/?v%3Dlui
I've used the basic html mode before when the normal (js heavy) one was a bit slow, and it ended up being much faster even if every action requires a whole new page request/render.