80-90% of the end-user response time is spent on the frontend, so it makes sense to concentrate efforts there before heroically rewriting the backend. Take a shower before buying a Porsche, if you know what I mean. Steve Souders, author of High Performance Websites and Yslow, has ten more best practices to speed up your website:

Update: Nice explanation in The importance of bandwidth versus latency of how long latencies cause cascading delays in resource loading. Doloto tries to optimize how resources are loaded.

Twenty new rules have been added to the original 14 rules for sizzling web performance. Part of scalability is worrying about performance too. The front-end is where 80-90% of end-user response time is spent and following these best practices improved the performance of Yahoo! properties by 25-50%. The rules are divided into server, content, cookie, JavaScript, CSS, images, and mobile categories. The new rules are:

It is fairly obvious that web site performance can be increased by making the code run faster and optimising the response time. But that only scales up to a point. To really take our web sites to the next level, we need to look at the performance problem from a different angle.

This is what Mike Peters says he can do: make your site run 10 times faster. His test bed is "half a dozen servers parsing 200,000 pages per hour over 40 IP addresses, 24 hours a day." Before optimization CPU spiked to 90% with 50 concurrent connections. After optimization each machine "was effectively handling 500 concurrent connections per second with CPU at 8% and no degradation in performance."

Mike identifies six major bottlenecks:

Cacti is a network statistics graphing tool designed as a frontend to RRDtool's data storage and graphing functionality. It is intended to be intuitive and easy to use, as well as robust and scalable. It is generally used to graph time-series data like CPU load and bandwidth use.

The frontend is written in PHP; it can handle multiple users, each with their own graph sets, so it is sometimes used by web hosting providers (especially dedicated server, virtual private server, and colocation providers) to display bandwidth statistics for their customers. It can be used to configure the data collection itself, allowing certain setups to be monitored without any manual configuration of RRDtool.

Update: Speed up Apache - how I went from F to A in YSlow. Good example of using YSlow to speed up a website with solid code examples.

Every layer in the multi-layer cake that is your website contributes to how long a page takes to display. YSlow, from Yahoo, is a cool tool for discovering how the ingredients of your site's top layer contribute to performance.

Cal Henderson writes at thinkvitamin.com: "With our so-called "Web 2.0' applications and their rich content and interaction, we expect our applications to increasingly make use of CSS and JavaScript. To make sure these applications are nice and snappy to use, we need to optimize the size and nature of content required to render the page, making sure we’re delivering the optimum experience. In practice, this means a combination of making our content as small and fast to download as possible, while avoiding unnecessarily refetching unmodified resources." A lot of good comments too.