Network RTT

Why It Matters

• It multiplies across every request. A page might make 50 or more network requests during loading. If each round trip takes 200 milliseconds, that latency compounds quickly. Reducing RTT speeds up the entire loading waterfall.
• It hits mobile users hardest. Mobile connections typically have higher latency than wired broadband. Users on cellular networks often experience RTTs of 100–300 milliseconds or more, making every request noticeably slower.
• It cannot be eliminated by faster servers alone. Even if your server responds in one millisecond, the physical distance between the user and the server adds latency. A user in Tokyo requesting files from a server in New York will always have higher RTT than someone nearby.
• It affects perceived speed directly. Users feel RTT as the gap between clicking and seeing a response. High latency creates a "nothing is happening" moment that makes people impatient, even if everything is working correctly on the server side.

How to Reduce It

1. Use a CDN. A content delivery network places copies of your files on servers around the world. Users download assets from the nearest location, dramatically reducing the physical distance and therefore the round-trip time.
2. Reduce the number of requests. Fewer requests mean fewer round trips. Combine CSS files, bundle JavaScript, use image sprites or inline small assets, and eliminate unnecessary third-party calls to minimize total request count.
3. Enable connection reuse. HTTP keep-alive and HTTP/2 multiplexing allow multiple requests to share a single connection, avoiding the overhead of establishing a new connection (and its associated round trips) for every file.
4. Preconnect to critical origins. Use <link rel="preconnect"> to establish connections to important third-party domains early. This gets the DNS lookup, TCP handshake, and TLS negotiation out of the way before the browser actually needs the resources.
5. Cache aggressively. Resources that are cached in the browser skip the network entirely — zero round trips. Set long cache lifetimes for static assets and use cache-busting filenames when content changes.

Common Mistakes

• Ignoring geographic distance. Hosting everything on a single server in one region means users on the other side of the world experience high latency on every request. A CDN solves this without changing your application code.
• Loading too many third-party scripts. Each third-party domain requires a separate connection setup — DNS lookup, TCP handshake, TLS negotiation — all adding round trips. Every script from an external origin increases total RTT cost.
• Not using HTTP/2. HTTP/1.1 limits parallelism and forces browsers to open multiple connections. HTTP/2 multiplexes many requests over a single connection, significantly reducing the number of round trips needed.
• Only optimizing file sizes. Making files smaller is great, but if each file still requires a full round trip to a distant server, the latency remains. You need to reduce both file sizes and the number and distance of round trips.