Gmail used to offer a low bandwidth / performance webmail interface, that was essentially their original UI. Ran like greased lightning, used barely any memory. Emails loaded almost instantly.
Isn't it still the case then? I used the basic HTML version when I was working at Google to try to understand whether or not it was slow because of the (unoptimal) frontend or not (it was the backend that sometimes took >=600ms to load messages unfortunately, not the frontend).