An Event Apart: Performance as User Experience

by Luke Wroblewski April 3, 2018

In his Performance as User Experience presentation at An Event Apart in Seattle, Aaron Gustafson shared a number of ways to optimize Web page performance. Here's my notes from his talk:

  • Our jobs as designers is to reduce friction for our users. Poor performance causes friction and can negatively impact key metrics like conversion and revenue.
  • How do Web pages load: when you enter an address in a URL bar, a DNS look-up replies with an IP address, then there's a TCP handshake followed by the actual request for files/data, once there's a response the browser can actually do something.
  • Once the browser gets a response, it can assemble the document object, the render tree, layout, paint, and finally load. CSS and Javascript can delay this process and sometimes cause it to run again.

Steps for better performance

  • Use native features whenever possible. They are effectively free. Semantic elements not only reduce bytes but also contain attributes that provide a lot of functionality like placeholder, autocomplete, autocorrect, type, etc. System fonts can help reduce the need for custom font downloads. Font stacks can cover fallback issues.
  • Only include assets you actually need. Every tool has a cost, make sure you are really using enough of each tool to justify that cost. Chances are the tool you are trying to use is already on a CDN and you can use resource hints to speed up the download process. But downloading isn't everything as Javascript frameworks also take time to execute, which is slower than native JS processing.
  • Optimize everything. Task runners like Grunt and Gulp can help automate optimizations of Javascript, CSS and HTML. Minify and pre-compress all your files.
  • Think about when you load assets. If you have Javascript files divided into modules, you can defer functions you won't need until the after the DOM is loaded. The async attribute also allows you to load files when it makes the most sense. Just make sure you don't hit any race conditions if some of your Javascript files have dependencies on others.
  • Why so many different files? Under HTTP1, each resources was requested sequentially. Now with HTTP2, you set a single connection and then stream in resources as needed.
  • Consider how you load assets. Start simple by loading just your default (often mobile) CSS file. You can add a media query as a threshold for loading more advanced CSS in browsers than can render it. Conditional comments (which only work in IE8 and below) can either load or hide elements from older browsers. Similar techniques can be used to conditionally load images and animations (via SVG support).
  • Only load assets when they add value. Not every article needs an image, think twice before you include it. Images are 53% of the average web page and very expensive size-wise. If you need an image, use the right format. GIFs ae good for solid colors, JPGs for photographs, PNGs are JPG alternatives with alpha transparency, WebPs are note well supported but optimized in many ways.
  • Images can be optimized by removing color, blurring parts of images, resizing, compressing, and using appropriate formats. We can use the picture element to add WebP images in browsers that support them. Remember to put your smallest files first, because the first one that works is what gets used.
  • Every choice we make affects our users’ experiences. Let’s spend our time to save it for our users.