I have found myself reading quite a bit about usability and web design recently. While I have learned a lot about design I have also learned that there is a large portion of the design community which is not terribly familiar with how they can improve performance of the websites they are creating. While a pixel perfect layout is a beautiful thing, no one wants to wait 20 seconds for it to load. Since the web design community has taught me so much, I wanted to give back by writing this post on some simple techniques for improving site performance.

Welcome to Web Performance 101, here is your first assignment:

The above is a waterfall chart which shows how long the pieces of a particular website take to load. To be nice I've blanked out the website URL's but I left the file extensions since they will come in handy later. What this chart shows us is that initially the browser made a request for the index ("/") page and 1342 ms later it had received all of HTML for that page. One second in the browser makes a request for the first javascript file which takes 585 ms to load. So on and so forth for about five seconds. While there was more to the chart I cut it off for brevity.

Progressive Rendering

You probably have noticed that often times your browser starts drawing the website before the entire thing has loaded, this is called progressive rendering. This is really nice because if done correctly it can make your website "feel" much faster for end users. In order for the browser to start drawing the page it needs two things, the HTML and the CSS. Until the HTML/CSS are downloaded your users are going to be staring at a blank white screen. This is why it is really important to put CSS in the head of the document before javascript and images. In the chart above there is a vertical green line at about 2.25 seconds, that is when the browser could start rendering the page. Had the designer of the site put the CSS before those five javascript files the page could have started rendering at 1.5 seconds or even earlier. Its such a simple an easy change, that there really is no reason not to.

Javascript: The Performance Killer

While every website is going to have a waterfall pattern the goal is to have a very steep waterfall. The further the bars extend out to the right of the graph, the longer the site takes to load. Hence, we want to shoot for a very steep and short waterfall which should translate into a fast site. Unfortunately this chart looks more like stairs then a waterfall, the reason is javascript.

While it may not be obvious to you, browsers are pretty smart. For example, when fetching the images on a website most browsers can download more than one at a time. Older browsers (IE7, Firefox 2) usually download two items at a time and newer browsers (IE8, Firefox 3) download 6 or more at a time. Notice how there is overlap between items 8 & 9 as well as 10 & 11, those items were being downloaded in parallel. Then notice how there is no overlap between items 2-7. This is because unlike images or CSS, javascript blocks the browser from downloading anything else. While newer browsers tend to do a better job at this and download them parallel, most of the world is still running IE7 which is what was used to generate the above chart.

One way to improve this would be to take all the separate javascript files and combine them into a single larger file. This will reduce a lot of the overhead incurred by download many files. Note how most of the bar of the javascript file is actually green, that is "time to first byte." It is the time between when your browser say "Hey give me foo.js" and when it finally receives the first byte of that file. You can barely even see the blue lines at the end which is the time spent downloading the actual data. By combining those javascript files together you cut out most of the time spent waiting for the data to come back. There are many tools online and available for download which will automatically concatenate the files together for you.

Zipping it Up

The easiest way to ensure that your site loads fast is to send less data. Less data means less time required to download it which means better performance. This is where data compression comes in. Most big web servers provide the ability to compress the data it sends out into a much smaller format so that it can be transferred more quickly. Then once it reaches the browser it is smart enough to know that it is compressed and it will uncompress it and read it just the same way it normally would. The most popular compression algorithm used today is called GZIP, and it can often cut file size in half or even a third. It is important to note that this compression is only effective on text data, so you want to gzip your HTML, CSS, and Javascript files. You don't want to gzip images since it usually eats up more server CPU then its worth. The best thing about gzip is that it only requires a few extra lines in the server config file and then your done. This website has a good guide for how to enable it for Apache.

As I mentioned above it is often a good idea to combine all of your javascript files into a single file, the same holds true for CSS. So if you have your css split across many files, combine it into a single file so that it loads faster. An extra bonus to this is that compression rates tend to get better as files get larger. Thus while the amount of text is still the same you will probably end up transferring less data and skipping much of the time wasted waiting for the data to arrive.

That's where we will stop for today. Hopefully you learned a few things about web performance which you can apply to websites you are working on. Keep in mind there are a lot more tips and tricks for improving performance, this is only the tip of the iceberg.