User Research on ‘Data saving’ mode

we're thinking of introducing a data saving (lite) mode to our app, after having some feedback from our users. But as a UX designer I'd like to do some research first and understand the market and maybe what other apps are doing.

Also I'd like to go to our beta and testing groups for their input. But I need some help around what sort of questions I can ask.

I know what our goal is but don't want to design something purely on our assumptions. Can I get some ideas and suggestions around what I could ask to our users so I understand if this a feature they'll need or are just making wild guesses.

EDIT: this is a data saving mode so it'll reduce the use of data so pics, videos won't load. not to be confused with offline mode.

Thanks everyone