As someone with recent experience using a relatively slow Android phone, it can be absolutely brutal to load some web pages, even ones that only appear to be serving text and images (and a load of trackers/ads presumably). The network is never the bottleneck here.

This problem is compounded by several factors. One is that older/slower phones cannot always use fully-featured browsers such as Firefox for mobile. The app is takes too many resources on its own before even opening up a website. That means turning to a pared-down browser like Firefox Focus, which is ok except for not being able to have extensions. That means no ublock origin, which of course makes the web an even worse experience.

Another issue is that some sites will complain if you are not using a "standard" browser and the site will become unusable for that reason alone.

In these situations, companies frequently try to force an app down your throat instead. And who knows how much space that will take up on a space-limited device or how poorly it will run.

Many companies/sites used to have simplified versions to account for slower devices/connections, but in my experience these are becoming phased out and harder to find. I imagine it's much harder to serve ads and operate a full tracking network to/from every social media company without all the javascript bloat.