Improving page rendering performance

When we think about page speed there are actually two different scenarios that we need to address:

The speed of initial page load and possible reloads after that
The speed of changing the page within Single page application (SPA)

The first one is usually a slower process. A browser needs to load all the HTML, CSS, JavaScript, and images - and then it needs to understand and execute those files, calculate layout, paint components and finally composite the whole view. The initial page load is the slowest since the consequent page reloads can take benefit from browser caches.

SPAs can improve from that since they don't necessarily need to download anymore JavaScript, HTML, or CSS - already downloaded JavaScript might be enough for rendering consequent pages when a user wants to navigate to another page. Most of the time SPAs just fetch data for that page.

These two UX scenarios might also conflict with each other. If all the JavaScript is in one big bundle, page changes within a SPA are fast. However, downloading and evaluating a big JavaScript file is slowing initial page rendering down. Even though users rarely experience the full initial page load speed when they use an SPA like Flex Template for Web, it is good to keep track of that speed. Especially since that is what search engine bots are experiencing and therefore it might affect your page rank.

Check page performance

The first step is, of course, to start measuring performance. Lighthouse is a good tool to check rendering performance. At least check those pages that are visible to unauthenticated users (e.g. landing page, search page, listing page, about page and other static pages).

Lighthouse will give you some tips about how to improve performance and other aspects that website developers should think about.

Optimize image sizes

If your page is showing images, you should check that the image size is not bigger than what is needed. So, adjusting image dimensions is the first step, but you should also think about image quality, advanced rendering options and possibly serving those images from CDN instead of from within your web app.

Quick checklist:

Check that the actual dimensions of an image match with DOM element's dimensions.
Lighthouse suggests that image compression level should be 85% or lower. Read more
Good rule-of-thumb is that use JPEG for images and photos, where PNG is better for graphics, such as logos, graphs and illustrations.
If you are using JPEG images, think about saving them as progressive JPEGs. Read more + Photoshop guide
If you are using PNG images, consider running them through PNG optimizers to reduce file size. Plenty of options available, one example is TinyPNG.com
Think about serving images and other static assets from some CDN. Read more.

Lazy load off-screen images and other components

Another way of dealing with images is to lazy load those images that are not visible inside an initially rendered part of the screen. Lazy loading these off-screen images can be done with helper function: lazyLoadWithDimensions (from util/contextHelpers/). Check SectionLocations component for details.

Use sparse fields

Another way to reduce the amount of data that is fetched from API is sparse fields. This is a relatively new feature and Flex template app has not yet leveraged it fully, but it is created to reduce unnecessary data and speed up rendering. You can read more from Flex API docs.

About code-splitting

We haven't yet implemented code-splitting to template app. Our current setup is creating one UMD bundle file that is used on both client-side and server-side rendering (SSR). Unfortunately, Webpack (a bundling tool used by sharetribe-scripts dependency) has a bug that prevents using code-splitting. This means that there need to be separate builds for browser and SSR for code-splitting to work.

In practice, it is probably best to add another Webpack build configuration for the server in Create React App (CRA) fork (sharetribe-scripts is a fork of CRA repo). That will also mean some refactoring in server/ folder as well as in app.js file.

Furthermore, if you are considering code-splitting, you should also be aware that there might be a problem with a "back" navigation button. (If the page loads asynchronously, a browser might not be able to return the user to the correct scrolling position on the previous page.) We would also like to be aware if users of the template app are considering code-splitting - if there's more demand for this feature, we should prioritize it accordingly.

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