Website performance is a daunting, complicated subject; everything from servers, networks and the code itself affects the length of time it takes for our carefully-crafted pixels to arrive on the screen of our viewer. However, when it comes to WordPress themes, there are a few simple guidelines we can follow to make sure our themes help, rather than hinder, that process.
There is a single, unifying concept behind all of the following practices: less is more. Performance can be improved every time we:
- reduce the amount of data we fetch,
- reduce the time required to fetch that data, and
- reduce the number of times we have to fetch data at all.
Images require special treatment to not get in the way of performance. These tips will make sure any images you require are as lean as possible.
- Many images can be further compressed before suffering any visible loss in quality. Make sure any included images in your theme are compressed, either with your favourite image editor, or a tool like PNGCRUSH. Use the image format (JPG, PNG, or GIF) that best suits your situation and results in the smallest file size.
- The best way to reduce image size? Don’t use them at all. If all you need are icons, use an icon font (like Genericons) instead, or the vector format SVG. Use CSS whenever possible for graphic elements, such as spinners or loaders.
- If you must use multiple small images, combine them into a single file as CSS sprites, to reduce their download to just one HTTP request.
Scripts and Stylesheets
Scripts and stylesheets add up quickly. Simply adding third-party scripts and libraries that cover all edge cases can result in a lot of un-necessary code, while comprehensive CSS frameworks can also amount to needless data. Keeping in mind our mantra of less is more, two further action points can bring significant improvements to our page loads.
- Minify, combine and compress your scripts and stylesheets to reduce their size and the number of requests to load them.
- Enqueue scripts and stylesheets only on the pages that require them. See the Twenty Fourteen default theme for an example of how CSS and JS for the slider are conditionally loaded for just the home page.
Web servers have a very repetitive job. If ten users request our website in one minute, and nothing has changed on that page in that minute (a new post, categories removed, menus changed), it’s a bit of a waste for the web server to keep querying the database for the same information every time. This is where caching comes in. Caching is the general idea of doing the time-consuming data fetching one time, storing the results, and then delivering those stored results the next time someone asks for that same page. While the details of caching are far beyond this article, suffice it to say that the more our theme code can take advantage of caching, the speedier our sites will be – and the Transients API is a simple tool for theme developers to do just that.
Theme developers can store long-running queries in a transient, meaning each time that information is requested by the theme, it will be delivered quickly from cache rather than from a full query of the database. Secondary loops can be stored in transients, as well as the results of uncached core functions. The Twenty Fourteen default theme uses a transient for handling Featured Content.
For more information on transients and caching in general, Zack Tollman of The Theme Foundry has written an easy-to-understand overview of how WordPress uses caching in core.
Google’s PageSpeed Insights tool is extremely helpful when measuring performance of your sites in general. While some of it will not apply directly to theme code, it can point out issues with large images, unminified assets and more. There are two versions: a web version and a Chrome extension. The web version is usually more up-to-date (and now includes mobile-specific tests), while the extension can be used for offline, local testing.
Pingdom offers a suite of tools for web developers, one of which is the Website Speed Test. While PageSpeed is an overall review of best practices, Pingdom’s page test is focussed on load time, with the number of requests and load size for context.
Getting content in front of our users as fast as possible is more important than ever these days, particularly considering the growing use of mobile phones and tablets. If we want content in front of our user within one second, we need to squeeze every millisecond we can get out of our themes and WordPress installs. Performance is an ongoing, incremental process which, with these guidelines and regular education, will keep your themes in top shape for all of your visitors, mobile and up. Remember: less is more!