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.

It was nice while it lasted.

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).