SpeedCurve is built on top of the leading open source web performance testing framework WebPageTest and our metrics are the same as what you're used to seeing in WebPageTest:
Sometimes also called "First Byte". The time from the start of the initial navigation until the first byte of the base HTML page is received by the browser (after following redirects). This is a good approximation of the time it took your server to generate the HTML on the server and then deliver it over the network to the user's browser.
The Start Render time is measured as the time from the start of the initial navigation until the first non-white content is painted to the browser display. Any CSS and blocking JS you have on the page has to be downloaded and parsed by the browser before it can render anything to screen. This is called the critical rendering path and the Start Render metric is very important in understanding how long users have to wait before anything is displayed on screen.
Visually complete is the time at which all the content in the viewport has finished rendered and nothing changed in the viewport after that point as the page continued loading. It's a great measure of the user experience as the user show now see a full screen of content and be able to engage with the content of your site.
The Page Load time is measured as the time from the start of the initial navigation until the beginning of the window load event (onload). While Page Load can be a useful metric it can also be deceiving as depending on how the page is constructed it doesn't always represent when content is rendered to screen and the user can interact with the page. Unfortunately many organizations and other monitoring tools still default to reporting Page Load as an important performance metric. It's in no way a good measure of the user's experience and something the industry needs to move on from.
Moving beyond window.onload()
The Speed Index is the average time at which visible parts of the page are displayed. It is expressed in milliseconds and dependent on size of the view port. It represents how quickly the page rendered the user-visible content (lower is better). SpeedIndex is often the metric we show by default as it best represents the user's experience as the page rendered over time from starting completely blank to the viewport being visually complete. You can find out more about the details on how WebPageTest calculates SpeedIndex below.
Speed Index – how it works and what it means
The Google PageSpeed Insights score is a number out of 100 based on a set of best practise coding rules developed and updated by Google. A score over 85 indicates a page that is performing well.
Google PageSpeed Insights