This could be implemented at the site level (unfortunately with bloated frameworks, few care). One possible implementation could have the browser measure how long it takes to load assets and switch to requesting lightweight ones if loading takes too long.

Oftentimes, the real killer is not so much bandwidth but latency. If a page requires many requests to complete sequentially (ie. because one asset needs to be loaded in order to know how to request the next one), that all adds up real quick and produces a poor user experience.

> switch to requesting lightweight ones if loading takes too long

Why not start with downloading the lightweight ones, and progressively load more?

I.e. first download what this text-based browser would download, display it, and then (optionally) fetch the rest.