The Waterfall is the most complex section on GTmetrix, however, learning how to read it will provide you with extremely valuable information. It displays the request-by-request loading behavior of your website. Every script, media file, and/or third-party resource your page requests are displayed on Waterfall.
It is particularly useful for developers as it visualizes the loading behavior of websites. Each website element is indicated on the main graph with the exact time it took to load:
Look out for any errors like 404 or 403, as that might indicate an issue with your website which prolongs its loading time:
You can also click on waterfall itself to see more information:
Lastly, you can hover over the loading bar and see that moment’s CPU, memory usage, along with the upload and download speeds:
High CPU or Memory usage in the Waterfall might mean that you are exceeding the resources of your hosting plan. Make sure to check the Order Usage section in hPanel.
NOTE:
To know more about reading Waterfall we recommend checking the official GTmetrix guide - How to Read a Waterfall Chart for Beginners