Updated: Sun Mar 26 2017

Cheating For Website Performance

Frontend tips for speeding up websites

Wed Mar 11 2015

Have been turning into a bit of a performance nut lately. This is what I’ve found useful for speeding up websites. These are mostly frontend optimisations; I’m not going to delve into server performance here.


Minify Everything

By far the most important thing to optimise is images. There are great free tools like graphicsmagick that let you automatically compress images without visible quality loss, rescale to different dimensions, crop, etc. They can be a part of your standard build chain, so there’s absolutely no excuse for not using them. See example (scroll down to image processing).

Another important thing to compress is JavaScript. Modern JavaScript libraries (and hopefully your application’s code) tend to be richly commented, bloating the source size, with the expectation of being minified for production use. With massive frameworks like Angular, React, or Polymer, the total size easily rockets past a megabyte. Minification gets it down to manageable size.

Minifying CSS is usually less important, but like everything else, it’s a useful optimisation and there’s no excuse for not doing it.

Concatenate Everything

Network latency is a huge deal. I can’t stress this enough. Depending on the connectivity between your servers and your users, latency could range from 50ms to as much as a second.

If you serve assets as multiple independent files, the browser has to make separate network requests for each. Browsers only download a few assets at a time, stalling other requests, which means any additional, say, stylesheets delay the beginning of loading for other assets like images or fonts. Even when everything is cached and elicits a 304 “not modified” response, the browser still has to wait longer before rendering the entirety of the page.

That’s bad. To avoid that, make sure to concatenate assets used on each page, like stylesheets, scripts, and icons (see below on that).

Use Pjax

Update: see this in-depth post on pjax.

Pjax is a cheap trick that combines history.pushState and ajax to mimic page transitions without actually reloading the page.

The basic idea is dead simple and can be implemented in a few lines of code. Attach a document-level event listener to intercept clicks on <a> elements. If the clicked link leads to an internal page, fetch the page by ajax, replace the contents of the current page, and replace the URL using pushState. For browsers that don’t support this API, you simply fall back to normal page transitions.

Despite the simplicity, the benefits are stunning. It gives you most of the advantages enjoyed by SPA (single page applications). The browser gets to keep the same JavaScript runtime and all downloaded assets, including images, fonts, stylesheets, etc. This dramatically improves page load times, particularly on poor connections such as mobile networks. This also lets you maintain a persistent WebSocket connection while the user navigates your server-rendered multi-page app!

There are a few implementations in the wild, but they require clientside and server-side configuration. If you’re like me, this will seem like a waste of time. The biggest benefit of pjax is keeping the browsing session. Micromanaging partial templates is probably not worth your time, but everyone’s needs are different.

I wrote a simple pjax library that works with zero config. Check the gotchas to see if it’s usable for your site, then give it a spin or roll your own! The library is also used on this very site. Inspect the network console to observe the effects.

Use Server Rendering

There’s a trend towards single page applications (SPA) with clientside routing and rendering. They tend to skip server-side rendering in favour of being data-driven, usually through a RESTful API. As a result, they tend to have slow initial page loads. This is bad, particularly on slow connections, which is typical for mobile.

Practice has shown that for consumer-facing websites, initial load time matters. On top of that, lack of prerendering costs you SEO. Don’t fall into this trap; server rendering is a sacrifice you don’t have to make. Some JavaScript UI libraries, like React, already support isomorphic routing and rendering, and other frameworks, like Angular 2 and Ember, are planning to support it. Make sure to research this feature for your stack of choice.

Make Your JavaScript Lazy

If your application is JavaScript-heavy, you should use a module system with lazy loading. This is supported by the ES6 module system, and you can use it today with SystemJS and, optionally, jspm. You can also achieve a similar effect with AMD.

The core parts of the application should be bundled into a single file, and big but optional parts may be imported asynchronously when needed. If your app is small, you can skip lazy loading and bundle the entire app.

Use Font Icons or Inline SVG

Most sites need icons. In the past, we had to use raster images. However, in the days of widespread retina displays, @font-face, and SVG, that’s a poor option. Hopefully you have switched to the vector alternatives: icon fonts and SVG icons. They scale to any display sharpness and are easy to style with CSS.

SVGs can be embedded into the document or base64-encoded directly into your CSS, eliminating icon flicker on page load. They can also be directly manipulated with JavaScript for cool visual effects. On the other hand, icon fonts are easier to set up and use, and cost less bandwidth than embedded SVGs. For most sites, a mix of both solutions will probably be optimal.

Serve Static Assets

This goes without saying, but you should double check to make sure your server is properly configured for static files like images, stylesheets, and scripts. It should include headers that tell the browser to cache the file, and respond with 304 for unchanged assets. This eliminates a lot of redownloading, reducing latency+download time to latency+0.

Reduce Latency

Network latency is a huge deal. It’s a part of each request made by the browser, even for static assets with 304 responses. The browser blocks page rendering while downloading the document and anything included in <head>, which defines how snappy or sluggish your site feels. The browser may also wait for the first few images (Firefox seems to have this tendency), or it may choose to render the page and later flicker them into view, and latency determines how quickly this happens.

On many sites, the document is rendered dynamically and involves database access. This absolutely needs to be fast, but this work is usually done once per page load. The rest comes from network latency for the document and assets. Make sure to use a web hosting with low latency times for your target audience. If your audience is all over the world, pick a server with good average latency and use a caching proxy / CDN like CloudFlare to reduce latency for static content.

Consider a Static Website

Simple websites with one maintainer, like a personal page or a blog, don’t need a scripting engine with a database. You can prerender them into HTML files, then serve with nginx or on a service like GitHub Pages. Dynamic functionality can be implemented with ajax.

Serving static files is naturally more performant than rendering templates on each request. They’re also automatically subject to caching. When the base document is cached, some browsers may serve the entire page, including assets, from the cache, rendering it with zero latency.

Static site generators are plentiful, and if they don’t float your boat, you can write your own in an afternoon.