And yes, that makes a difference, you can deliver more features, faster, more reliably.
But, if those sites just bog down and take double-digit seconds to load, even when properly deployed on scaleable delivery architectures, with fiber-optic speeds, and CAD/gamer-level power machines, they are junk.
And I increasingly see exactly this junk been over the last few years. Even (and often especially) the major sites are worse than ever. For example, on the above setup, I've reverted Gmail to the HTML-only version to get some semblance of performance.
Sure, some of this could be related to Firefox' developments to isolate tabs and not reuse code across local containers and sessions, but expecting to get away with shipping steaming piles of cruft because you expect most of it will be pre-cached is no excuse.
Your site might have the look and features of a Ferrari, but if it has the weight of a loaded dump truck, it will still suck. If you are not testing and requiring good performance on a rural DSL line and mid-level laptop (or similar example of constrained performance), you are doing it wrong.