This chapter is from the book
This chapter is from the book
This chapter is from the book
Rule 5—Reduce Objects Where Possible
Web pages consist of many different objects (HTML, CSS, images, JavaScript, and so on), which allows our browsers to download them somewhat independently and often in parallel. One of the easiest ways to improve Web page performance and thus increase your scalability (fewer objects to serve per page means your servers can serve more pages) is to reduce the number of objects on a page. The biggest offenders on most pages are graphical objects such as pictures and images. As an example let's take a look at Google's search page (www.google.com), which by their own admission is minimalist in nature.7 At the time of writing Google had five objects on the search page: the HTML, two images, and two JavaScript files. In my very unscientific experiment the search page loaded in about 300 milliseconds. Compare this to a client that we were working with in the online magazine industry, whose home page had more than 200 objects, 145 of which were images and took on average more than 11 seconds to load. What this client didn't realize was that slow page performance was causing them to lose valuable readers. Google published a white paper in 2009 claiming that tests showed an increase in search latency of 400 milliseconds reduced their daily searches by almost 0.6%.8
Reducing the number of objects on the page is a great way to improve performance and scalability, but before you rush off to remove all your images there are a few other things to consider. First is obviously the important information that you are trying to convey to your customers. With no images your page will look like the 1992 W3 Project page, which claimed to be the first Web page.9 Since you need images and JavaScript and CSS files, your second consideration might be to combine all similar objects into a single file. This is not a bad idea, and in fact there are techniques such as CSS image sprites for this exact purpose. An image sprite is a combination of small images into one larger image that can be manipulated with CSS to display any single individual image. The benefit of this is that the number of images requested is significantly reduced. Back to our discussion on the Google search page, one of the two images on the search page is a sprite that consists of about two dozen smaller images that can be individually displayed or not.10
So far we've covered that reducing the number of objects on a page will improve performance and scalability, but this must be balanced with the need for modern looking pages thus requiring images, CSS, and JavaScript. Next we covered how these can be combined into a single object to reduce the number of distinct requests that must be made by the browser to render the page. Yet another balance to be made is that combining everything into a single object doesn't make use of the maximum number of simultaneous persistent connections per server that we discussed previously in Rule 3. As a recap this is the browser's capability to download multiple objects simultaneously from a single domain. If everything is in one object, having the capability to download two or more simultaneous objects doesn't help. Now we need to think about breaking these objects back up into a number of smaller ones that can be downloaded simultaneously. One final variable to add to the equation is that part above about simultaneous persistent connections "per server, which will bring us full circle to our DNS discussion noted in Rule 4.
The simultaneous connection feature of a browser is a limit ascribed to each domain that is serving the objects. If all objects on your page come from a single domain (www.akfpartners.com), then whatever the browser has set as the maximum number of connections is the most objects that can be downloaded simultaneously. As mentioned previously, this maximum is suggested to be set at 2, but many browsers by default have increased this to 6 or more. Therefore, you want your content (images, CSS, JavaScript, and so on) divided into enough objects to take advantage of this feature in most browsers. One technique to really take advantage of this browser feature is to serve different objects from different subdomains (for example, static1.akfpartners.com, static2.akfpartners.com, and so on). The browser considers each of these different domains and allows for each to have the maximum connects concurrently. The client that we talked about earlier who was in the online magazine industry and had an 11-second page load time used this technique across seven subdomains and was able to reduce the average load time to less than 5 seconds.
Unfortunately there is not an absolute answer about ideal size of objects or how many subdomains you should consider. The key to improving performance and scalability is testing your pages. There is a balance between necessary content and functionality, object size, rendering time, total download time, domains, and so on. If you have 100 images on a page, each 50KB, combining them into a single sprite is probably not a great idea because the page will not be able to display any images until the entire 4.9MB object downloads. The same concept goes for JavaScript. If you combine all your .js files into one, your page cannot use any of the JavaScript functions until the entire file is downloaded. The way to know for sure which is the best alternative is to test your pages on a variety of browsers with a variety of ISP connection speeds.
In summary, the fewer the number of objects on a page the better for performance, but this must be balanced with many other factors. Included in these factors are the amount of content that must be displayed, how many objects can be combined, how to maximize the use of simultaneous connections by adding domains, the total page weight and whether penalization can help, and so on. While this rule touches on many Web site performance improvement techniques the real focus is how to improve performance and thus increase the scalability of your site through the reduction of objects on the page. Many other techniques for optimizing performance should be considered, including loading CSS at the top of the page and JavaScript files at the bottom, minifying files, and making use of caches, lazy loading, and so on.