Should users be prompted to determine bandwidth intensity of a site?

Serving up web content based on user agents alone seems exclusionary to me; mobile devices aren’t always constrained to low bandwidth.

From a UX perspective, would it be a bad strategy to present content optimised for low bandwidth by default and have users ‘opt in’ to downloading additional resources after initial load? For example, in a website context, this could mean loading in custom fonts vs. system fonts, presenting video instead of images, or downloading additional javascript for strictly presentational purposes.

To clarify, I’m not referring to responsiveness with regards to screen sizes across different devices, and it should be assumed the layout of key elements (navigation, calls to action, etc) is consistent across the low/high bandwidth versions.

The benefits include an optimised load time across all devices by default and relatively simple SEO (vs. breaking a site across multiple domains for mobile/desktop). But it’s not something you see a lot of in the wild, so I’d love to hear some thoughts.

EDIT

I've mocked up an example of what I'm leaning towards, you can view it at https://www.nickdawes.dev/portfolio. This implementation allows the user to opt in to downloading additional content, which in this case is a full-screen background video and custom fonts. If they opt out, no additional content is served.