Waterfall chart
The staircase of bars in DevTools, one row per request, showing when each file started, waited on the server, and finished.
See it
What it is
One row per request, time running left to right, each bar starting where that request started. It is the single most useful picture of a page load, and you get it free in the Chrome DevTools Network panel or in WebPageTest. Each bar is segmented: queueing and stalled, DNS, initial connection, TLS, request sent, waiting (that is TTFB for the file), then content download. The colors tell you whether a slow request was slow on the network or slow at the server.
Read the shape, not the individual rows. A fat first bar means the server is slow to answer. A neat diagonal staircase means each request only got discovered after the one above it finished, a critical request chain you break with resource hints or by inlining. A big empty gap before your hero image means late discovery, which is what preload and fetchpriority are for. A wall of bars that all start at once and crawl means you are bandwidth bound, so ship fewer bytes.
The gotcha: a waterfall recorded on your laptop on office wifi is fiction. Throttle the network and the CPU, disable cache for a cold-load run (then do a second warm run deliberately), and check a real device or a WebPageTest location before you believe the shape. Also read the time scale rather than eyeballing bar lengths: DevTools bars are positioned relative to the request start, and a short bar sitting far to the right is usually the real problem.
Ask AI for it
Walk me through this page load as a network waterfall. For every request give me start time, the queueing, DNS, connect, TLS, waiting (TTFB), and download segments, and the transferred size, sorted by start time. Then read the shape for me: flag any critical request chain where a request only starts after its parent finishes, any render-blocking resource in the head, any long stalls or gaps, and how late the LCP resource is discovered. Capture it with network and CPU throttling on and the cache disabled, and give me the three changes that would collapse the chain.