I don't think any of our apps are built with slow connections in mind at all.
Most of our web libraries and frameworks are indeed quite bloated (with features of convenience), downloading 20 MB of JS and 50 MB of content in total to render a page is insane when you think about it. We'd need to be able to turn off most images or visual elements to focus purely on the elements and their functionality, except for cases where displaying an image is critical for the function (and even then give the choice of showing a low quality version with a smaller file size). Things like web safe fonts that are already present in the browser/OS, most likely no libraries like React or Vue either (maybe Preact or Svelte).
We'd need to allow for really long request (say, fetch) timeout values, maybe even to choose how to set them based on the connection quality (if a user has a really fast connection but a request suddenly hangs and is taking upwards of a minute, something has probably gone wrong and it'd make sense to fail that request, vs a user in a remote area for whom all requests are similarly slow), assuming that the server doesn't mind connections that linger around for a long time at slow speeds.
We'd also need to allow for configuring an arbitrary HTTP cache/proxy for any site visited and file requested (say, store up to 1 TB on some local server based on file hashes and return the same for any user that requests that), but obviously things don't usually work that way over privacy/security concerns (nowadays different sites even download duplicate copies of the same files due to changes in the browsers: https://www.peakhour.io/blog/cache-partitioning-firefox-chro... ). Maybe even for any web request that the OS might want to do, like system updates, basically a full on MitM for the whole system.
Speaking of which, no more Electron or large software packages. Only native software with like Win32/WPF or maybe something like GTK/Qt, but nowadays it seems like even phone apps, not just desktop software, often don't use the system GUI frameworks, but instead ship a bunch of visual fluff, which might look nice and work well, but also takes up a bunch of space.
I don't think there are incentives out there to guide us towards a world like that, which doesn't quite make sense to me. Lightweight websites should lead to better customer/user retention, but in practice that doesn't seem like something that anyone is optimizing for - ads everywhere, numerous tracking scripts, even autoplay videos, for everything from news sites to e-commerce shops.
People who do optimize for that sort of stuff, seem to be a part of a smaller niche enthusiast community (which is still nice to see), like:
https://1mb.club/
https://512kb.club/
Admittedly, even I'm guilty of bloating my homepage size from ~150 KB to ~ 600 KB due to wanting to use a custom set of fonts (that I host myself), even WOFF2 didn't save me there.