Comment by 10000truths | Hacker News Reader

Comment by 10000truths | Hacker News Reader

The table is a bit misleading. Most of the resources of a website are loaded concurrently and are not on the critical path of the "first contentful paint", so latency does not compound as quickly as the table implies. For web apps, much of the end-to-end latency hides lower in the networking stack. Here's the worst-case latency for a modern Chrome browser performing a cold load of an SPA website:

DNS-over-HTTPS-over-QUIC resolution: 2 RTTs

TCP handshake: 1 RTT

TLS v1.2 handshake: 2 RTTs

HTTP request/response (HTML): 1 RTT

HTTP request/response (bundled JS that actually renders the content): 1 RTT

That's 7 round trips. If your connection crosses a continent, that's easily a 1-2 second time-to-first-byte for the content you actually care about. And no amount of bandwidth will decrease that, since the bottlenecks are the speed of light and router hop latencies. Weak 4G/WiFi signal and/or network congestion will worsen that latency even further.

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.

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.

For context this article was written when 95%+ of websites used HTTP 1.1 (and <50% used HTTPS).

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