Waterfall Chart (Network)

A waterfall chart is a diagram of a page load in which each resource is a row and its bar spans from request start to finish. Bars are segmented by phase: DNS lookup, connection, TLS, waiting for the first byte and content download.

Why it matters

Waterfalls make dependency chains and bottlenecks visible. You can see which request starts late because it was discovered late, which files block rendering, how long the server took to respond and where third-party requests pile up. Browser developer tools, WebPageTest and Lighthouse-based tools all produce one.

Example

How to read a typical pattern:

index.html     |==wait==|=dl=|
style.css                    |==wait==|dl|
font.woff2                              |==wait==|dl|
hero.jpg                      |====wait====|====dl====|

The font starts only after the CSS finishes, a chain. Preloading it would let it start with the CSS.

Common mistakes

Reading only total height. Long, narrow stacks of tiny requests and a few wide bars point to different problems.
Ignoring long green "wait" sections, which mean slow server response, not a large file.
Analyzing one run. Network variance means repeating tests.
Not noticing the vertical line marks for FCP and LCP.

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