I usually append /h/ to the URL (/mail/h/) and get a HTML version. It can work even without JS and it has classic design with small rows that works well on my small screen. No Material Design and no huge elements with large offsets.
Ive always loved Japanese website layouts. Content is beauty.
Actually Gmail's approach is sane. They just keep old UI working as it used to many years ago although the backend could have changed completely.
they probably have an api in which you can limit the results by category. As the old interface doesn't have any limiter implemented, they're getting all the results back.
you can also disable categories in the newer interface and the effect would be the same (everything in the inbox)
https://mail.google.com/mail/u/0/#inbox
where should I put the /h/? https://mail.google.com/mail/u/0/h/