Testing on simulated 3G: what actually changes

A simulated 3G test throttles the network and latency inside the browser, not the router: with 400 ms of RTT and 1.6 Mbps our home page moves from an LCP of 708-1,360 ms to 2,672 ms, while TTFB stays at 335 ms. The server is not involved: the 340 KB across 24 requests are what weigh.

Three profiles, three answers

TTFB barely moves between profiles: 335 ms against 352 ms, against 319-529 ms unthrottled. When TTFB holds and LCP triples, the time goes after the first byte, downloading CSS, JavaScript and images. That is the most useful diagnosis this test produces.

The command

const cdp = await page.createCDPSession();
await cdp.send("Network.enable");
await cdp.send("Network.emulateNetworkConditions", {
  offline: false,
  latency: 400,
  downloadThroughput: 200 * 1024,
  uploadThroughput: 100 * 1024,
  connectionType: "cellular3g",
});
await page.goto("https://example.com/", { waitUntil: "load" });

The code needs Puppeteer, Chrome and a page already open in page. downloadThroughput is in bytes per second, not bits: 200 × 1024 corresponds to 1.6 Mbps. You can set the same conditions by hand in DevTools, under Network, with the "Slow 3G" profile.

What the test does not say

Network throttling does not slow the processor, so a page that spends 1.2 s of JavaScript hydrating looks faster than it is on a real phone. Lighthouse mobile adds a four-times CPU slowdown on top of the network limit, and uses 150 ms of RTT instead of 400: its 3G is gentler on latency and harsher on the processor. A budget needs both, not one of the two.

The limitation to name

Two runs per profile is a small sample, on one machine and one network: absolute values move by hundreds of milliseconds. Real 3G has no constant bandwidth either, and packet queues change with the cell. The test compares two versions of the site under the same conditions; it does not predict one user's time.

Where to continue

The EneUI catalogue indexes 111,985 components, the hero blocks and the record scroll expansion hero, a hero that pays for every image on 3G. To see what must not load at once read lazy loading: when it makes things worse; to split the JavaScript, splitting the bundle; for the first byte, TTFB: what it measures; and for the comparison between architectures, frontend frameworks compared.