Improve Your Performance

Image Issues

Images can be too big. A 72x72 pixel icon could actually use a 1400x800 image. Maybe, a plugin failed you and requested an image that is too big to be optimal. The GTMetrix report does two great things: it shows you wish images need optimization and it will also generate optimized images for you. You can use those in lieu of your problem images.

Image format choices. Know the difference between GIFs (or is that GIF?), JPEGs, PNGs, SVGs and bitmaps. Use the ideal image for the role in the presentation.

Javascript Placement

The header is the best (except when it isn't). When Javascript code goes into the header, that code executes first. It will block the display of content until the Javascript fully executes. This is called "render blocking." From an end-user perspective, this is horrible. They will be left with a white screen for much long than is ideal.

Put some stuff into the Footer. Some code can always go into the footer: analytics code, helper code, code relevant to the lower sections of the page (eg. a dynamic footer).

Client side Caching

Make content cacheable. WordPress will put query strings onto the tail end of some URLs and change that string with each load to give you a new and fresh load of that image, script or styling. From an end user perspective, that takes a lot of time.

Change the Expiry date (it's a meta data setting in the served assets) to push it days or even months into the future. That let's a web browser shoulder some of the load and keep copies of those elements on their computer instead of downloading them new with each page load.

Third Party Assets

Easy to deploy third party code almost always kill performance. While it's handy to drop in a snippet from Instagram, Facebook, Google, Click-to-chat, CRM tools, those can kill performance.

Pull as much as possible into your control. Download Google fonts and serve them from your site or your CDN.

Deliver the content from server side generated HTML, not Javascript delivered / built code.

Elect to leave out some of the third party assets on some pages. If performance is really important, be cautious of third party apps.

Compression

File downloads are a big time suck.

Fewer files to request and smaller files are key.

Each plugin and theme is likely to introduce several CSS and JS files apiece. This can quickly add to up to 20+ CSS and 20+ Javascript file requests.

Plugins like AutoOptimize can consolidate the CSS into fewer files and it can do the same with JavaScript.

Almost every web server is capable of serving out files compressed with GZip. Almost every web browser is capable of decoding GZipped files.

Server Caching

Store static copies of complex output (eg. leaderboards, lists of recent posts, stats).

Optimize code on the server before it is shipped to the client.

Deploy a plugin like W3 Total Cache, WP Cache, etc.

Keep-Alive: Time is lost when some browsers reacquire the path to your server with each file request. By tweaking the server's Keep Alive, you can keep that connection active.

Content Delivery Networks (CDNs): Use a CDN to stow and deliver some of the content.
A CDN can take the pressure off of the server.
It can deliver content from a server closer to the end client.

Flush Your Output

Hang time is a killer when you're waiting for a web page. This can be a side effect of the foundations of a website-- PHP: the driver for WordPress and so many other website applications. By default, PHP can wait a long time before delivery of content back to the end user. It is possible to push it to begin earlier than otherwise. That sends content back to your audience faster. I do this by putting the flush() call at the top of the page.tpl.php file. If you're about to output the themed data, you're ready to deliver content, so I say push it out as soon as you can, even starting the process before the page.tpl.php is populated and served. One thing to note: flushing content may not be a tactic that plays well with GZip.

Aggregate CSS and Javascript

You should collapse all of your Javascript and CSS into one file each (all CSS in one; all Javascript in the other). While Drupal does ask for a lot supporting files, it compensates by allowing for the aggregation of Javascript and CSS into collected files. In extreme cases, this knocks down the number of files from 30+ files down to two to four files. If you're using a CMS make sure it can aggregate these supporting files. If you're doing a manual web design, make sure you follow this rule as closely as possible.

CSS Sprites

CSS sprites use portions of a larger image to fulfill some graphical need on your web page. Spriting isn't a new concept-- I probably built my first sprite for a video game over 25 years ago. But its role in web design is comparatively new.

In pursuit of fewer file downloads, you can lump multiple elements into a single image and then use CSS to slice that image for use. There is a lot of finesse to how you slice up an image with CSS. You need to pay attention to how the image will be used and you need to be comfortable using CSS backgrounds with cropping and the repeat concepts figured out.