"If the client is fast enough not to notice -- if not, of course, you could lose customers"

This.

I honestly think most front end devs don't test with anything except a fast PC (or Mac). They sure don't test with last year's phones. Web pages are getting more and more sluggish (generally speaking) all the time.

It's one thing to always want to work with new technology, but devs really need to spend some time in their customers' shoes.