A checklist to make sure you website will be fast!

• critical link (aka css) tags are in head
• less critical link tags are end of body
• less critical link tags lazy load
  • <link rel="preload" as="style" onload="this.rel='stylesheet'" id='dashicons-css' >
• JS loads with the async property
  • <script async src="https://hi.js"></script>
  • or defer when scripts need to be loaded in order, or require the DOMContentLoaded Event

• Always use next gen formats
  • webp -> chrome/firefox
  • jpeg xr -> ie11/edge
  • jpeg 2000 -> safari
• use jpg for photography, not png
• Size images properly
• Use srcsets for multiple image sizes
• Use the <picture> element to let the browser select the right image for a scenario
• Lazy load images below the fold

• Fonts should always load woff2 first
• woff for fallback
• You can use font-display: swap; to allow the browser to use a fallback font while custom font files are being downloaded.
• eot, or truetype is only needed for IE < 10

• Avoid expensive selectors when possible
  • border-radius
  • box-shadow
  • transform
  • filter
  • :nth-child
  • position: fixed;
  • Partial matching: [class^="wrap"]
• Don't use universal selectors
  • Universal selectors like *, [disabled], [type=“text”], etc. are very expensive for the browser to match, as every element in the DOM must be checked.
• Avoid deeply nested dependent selectors
  • The descendant selector is very costly, as the browser must check for a match with every descendant element. On a complex web page, this can result in thousands and thousands (perhaps even more) of descendant selector searches.
• Use media queries to load files based on use case

<link href="style.css"    rel="stylesheet" media="all">
<link href="portrait.css" rel="stylesheet" media="orientation:portrait">
<link href="print.css"    rel="stylesheet" media="print">
<link href="desktop.css"    rel="stylesheet" media="(min-width: 720px)">

• Bundles should always be minified
• Bundles should have 0 comments, and all license text extracted to a separate file

• All assets should be fingerprinted
• All assets should have Cache-Control: max-age=365000000, immutable as a header
• Assets should be served over http/2
• Assets should only be served on a cookieless domain
• All files should be cached by a CDN
• Support Brotli compression
  • 15-30% smaller than gzip
• Compress with gzip, or zopfli as a fallback to brotli
• Do not ship unused css, js

• Use a service worker to cache assets
• Use a service worker to prefetch pages users will most likely navigate to next
• Support offline, and spotty networks

There's a few great tools.

• Google's Lighthouse is built into chrome, and will score your page's performance from 0-100
• You can use source-map-explorer to visualize what is in your bundles. I often use this tool to find duplicate dependencies in a bundle.
• Web page test is always good to see how your assets load in the browser.
• The Chrome User Experience is a massive dataset extracted from chrome to show what real users see for performance. Your domain likely exists in this report today. You can generate yourself a dashboard based off the data.