That shows it's not always the massive JS libs that are the problem. It's also a lack of knowledge and interest from those who build and maintain the site.
True, the QAs job is to test it delivers as requested, UX if it is usable by the end user, ops that is is not heavy on resources etc. But it is a joint responsibility between all and especially the developer and designer that they deliver quality and not cut corners.
If there is an ultimate responsible it would perhaps be the project/product owner who would decide what is important and how much time and effort they should spend on which parts. But still it is not just pass-the-buck, they are all responsible.
I guess the results would be very different when using 4g? and that the real problem is that the author is using a unrealisticly slow connection.
Also, it's a mistake for developers to assume that everyone is on 4g, and that those on 4g are experiencing good speeds.
Even in London, if you're at a train station that's hit by delays, connectivity grinds to a halt as everyone starts looking for alternative transport.
In the big cities in India, 4g is pretty much always like that.