Feel you. I live in an area with not even very slow internet, just not full 4G speed, but the experience of navigating heavy-js websites is... extremly bad. Like often I try for 5 seconds and then just quit the tab - bad.

I feel like a lot of devs do not even try testing their websites / progressive webapps in slow speed situations or on low end devices. If they tried they would understand why here if you want people to try your service you must build an app, put it in the store and keep network messages at minimum. Webapps are a good way to get a "Not interested, thanks anyway"

Been a little while since I looked into this, but some devs do test for slow connection usability. The problem is that browser tools for simulating a slow connection don't drop and delay packets semi randomly like radio tech will. They just limit your max speeds.

Try Network link conditioner, built-in part of macOS and iOS. It includes options to set bandwidth, packet loss % and latency in each direction.

Iptables can be used to make a radio imitation.