As the author points out, it's not an issue of bandwidth so much as an issue of visual clutter and processor power—although watching a page try to make calls to a dozen separate web services before it will finish loading and let me interact with it smoothly is certainly frustrating. That's not necessarily the designer's fault, though, everyone shares the blame there.
No, the problem is that graphic designers generally approach web pages as if they were designing for print or television.
After many years working in large agencies in New York, I can count on one hand the number of "creatives" who knew any more about the technologies that make up the web than a little basic HTML 1.0, at best.
I started my career as a print designer in the 1990s. Designing for books and magazines, you had to understand your medium. Any print designer worth their salt regularly went on press checks, knew the difference between a sheet-fed litho and a web-fed offset press, how linescreens related to dpi measurements, how trapping worked, drum vs. flatbed scanning, all that.
Compare that to graphic designers for the web, who in my experience have little to no understanding of how a browser renders code, or how different services communicate.
The best graphic designers aren't trying to be programmers, but they know the medium they're working in, and they don't try to force it to be something it's not.
Beauty, usability, and performance aren't mutually exclusive concepts on the web. It's just that most clients only care about the first one (until the site has already launched), and most designers don't have enough knowledge of the second two.