Web - Timeline of a page load (Page Speed|Page Latency)

Page load is the latency performance metrics that cumulates:

• TCP latency (ie how fast, HTTP will be delivered)
• and HTML latency (ie how fast a page will be rendered)

HTML code is loaded by the browser in the order it appears in the file.

Load is not a single moment in time — it’s an experience that no one metric can fully capture. There are multiple moments during the load experience that can affect whether a user perceives it as “fast” or “slow”.

• Page Load - Navigation / Load event - The user begins a page load with a navigation.
• Phase: Browser - Page loading - Executing a HTTP request and getting the document
  • Page Load Metrics - Time to First Bit (TTFB)
  • Time To Response End (TTRE)
• Phase: DOM and CSSOM building
  • Event: DomContentIoaded event is fired (same as DOM - Document Loaded - onload event ?) when the document is loaded and parsed
  • Event: DOM - Ready event (Readiness|readyState)
  • Page Load - Content Load Time - The amount of time to fully load the HTML and build the DOM (external resources like images & stylesheets not yet loaded).
  • Page Speed - Load time - The amount of time to complete all the processing and resource (images, styles, etc) loading for the page.
• Phase: Browser - Rendering
  • Render tree: Building the render tree from the DOM and CSSOM
  • Box Model tree creation
  • Phase: Web Page - Painting
    • Page (Load|Speed) Metric - First Contentful Paint (FCP)
    • Page Load - Time To Interactive (TTI)

See DOM - Ready event (Readiness|readyState)

Webpack

• Page lifecycle API
• https://developers.google.com/web/updates/2018/07/page-lifecycle-api

Your web analytics provider should provide you with them.

Example for Google Analytics

• Javascript: HTML - Defer attribute - The defer attribute of a script will defer its execution at the end of a page load.
• Css and Javascript: preload
• Image: Web Resources - Lazy Loading

If JavaScript through the script element is placed before the body (without the HTML - Defer attribute attribute), it will therefore be loaded first and if it is supposed to affect the HTML in the body, it might not work, as the JavaScript would be loaded before the HTML it is supposed to work on.

Scripts in HTML have “run-to-completion” semantics, meaning that the browser will generally run the script uninterrupted before doing anything else (without the HTML - Defer attribute attribute), such as:

• firing further events
• or continuing to parse the document.

Probability of bounce by page latency.

(Slow pages lose user)

• 100 ms: user perceives as instantaneous
• 1s: is about the upper limit of human flow of though. User loses the feeling direct feedback
• 10s: Upper limit of attention. Feedback is important and the chance of context switch is high.

To record a page load, you would use a headless browser

• side-by-side-pageload.js - Load 2 or more pages side-by-side to visually see the difference in page load. Optional desktop viewport and throttling settings.

• https://developers.google.com/web/fundamentals/performance/rail
• Key Moments In Web Page Loading, Totally Tooling Tips (S3, E9)
• Progressive Web Apps with React.js: Part 2 — Page Load Performance
• See Page load processing model for HTML files