Welcome to the fourth part of our Web Performance Optimization series. It is our hope that these suggestions and best practices will assist you in improving the speed of your site. We view performance engineering as an iterative process whereby developers and testers will run load tests, analyze measurements, and tune the system incrementally. Our goal is to help you make your site faster and handle higher traffic.

We've talked extensively on this blog about how server applications can use caching to improve Web server performance, often dramatically. In addition to caching, Web site developers and administrators can employ other techniques to reduce the size of wire transmissions and increase document delivery speed.

File Consolidation

Web servers can reduce the number of requests generated by the client by reducing the number of separate files the client must fetch. Servers can facilitate this by combining separate files of the same type. From a maintainability standpoint, it often makes sense for a Web site developer to store the Cascading Style Sheet (CSS) code for her site in several separate files. A Web browser, however, doesn't care if the CSS code is contained in four small files, or one monstrous file; all of the elements share the same namespace once they're loaded into the browser.

According to The Exceptional Performance Team

80% of the end-user response time is spent on the front-end. Most of this time is tied up in downloading all the components in the page: images, stylesheets, scripts, Flash, etc. Reducing the number of components in turn reduces the number of HTTP requests required to render the page. This is the key to faster pages."

Minify is a PHP5 application that combines multiple Javascript and CSS files into a single file. This simple utility can eliminate anywhere from two to 12 HTTP requests for a single page. Minify goes the extra mile and applies GZip compression and cache control headers to these unified files for maximum performance.

Another key technique for consolidating multiple files is to take advantage of CSS sprites. This technique puts multiple images into one composite image that can be used by the browser for many different individual parts of a page. CSS background positioning is used to display only the image you need from the sprite.

The HTTP requests are greatly reduced because one request replaces potentially hundreds of requests. I have seen some customers' ecommerce pages that contain over 300 images. A sprite could produce a 300 to 1 reduction in requests for that page. Multiply that overhead savings by say 10,000 concurrent users, and the result is a tremendous performance improvement. Most new versions of browsers support CSS backgrounds and positioning, which has allowed developers to adopt this performance technique.