PageSpeed tells you a page is slow. WebPageTest tells you why. I use it on almost every technical audit to look at the waterfall and filmstrip.
What I look at:
– Pick a test location and connection close to the real audience, and a mid-range mobile profile. Testing on a fast desktop connection hides most problems.
– Read the waterfall top to bottom. Long gaps before the first byte point to the server, long chains of CSS and fonts point to render blocking.
– Use the filmstrip view to show clients exactly when the page becomes usable. It lands better than any metric.
– Run at least three runs and use the median, single runs are noisy.
Downside: some features, like certain experiments and history, now sit behind a login or paid plan, and the free queue can be slow at busy times. It is also lab data, so confirm with field data before promising improvements.
https://www.webpagetest.org/
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