[jiggawatts]

The reason why using a CDN is so effective for improving the perceived performance of a web site is because it reduces the length (and hence speed of light delay) of these first 7 round trips by moving the static parts of the web app (HTML+JS) to the "edge", which is just a bunch of cache boxes scattered around the world.

The user no longer has to connect to the central app server, they can connect to their nearest cache edge box, which is probably a lot closer to them (1-10ms is typical).

Note that stateful API calls will still need to go back to the central app server, potentially an intercontinental hop.

[10000truths]

Indeed, at some point, you can't lower tail latencies any further without moving closer to your users. But of the 7 round trips that I mentioned above, you have control over 3 of them: 2 round trips can be eliminated by supporting HTTP/3 over QUIC (and adding HTTPS DNS records to your zone file), and 1 round trip can be eliminated by server-side rendering. That's a 40-50% reduction before you even need to consider a CDN setup, and depending on your business requirements, it may very well be enough.