Earlier quoted context omitted.
Just throttle your router settings for download/upload - have fun! In all seriousness though, people designing websites should be given the shittiest connections possible and old computers (between 5 and 10 years old). Maybe then we'll end up with websites that haven't got more JS than content, broken CSS and images that push the page download over several megabytes.
Chrome Dev Tools let you simulate shit internet connection (2G, slow 3G, fast 3G, etc.) and Google encourages to this for development for the reasons you've mentioned.
Installing NoScript and blocking external CSS was the best decision I ever made. Now use The UK Government site team is my go to for a good web design philosophy: https://www.gov.uk/guidance/government-design-principles (Hell, if they could run the actual Government like this we would be in a lot better shape!)