Compress components with gzip
- Compression reduces response times by reducing the size of the HTTP response. Gzip is the most popular and effective compression method currently available and generally reduces the response size by about 70%. Approximately 90% of today’s Internet traffic travels through browsers that claim to support gzip.
Use cookie-free domains
- When the browser requests a static image and sends cookies with the request, the server ignores the cookies. These cookies are unnecessary network traffic. To workaround this problem, make sure that static components are requested with cookie-free requests by creating a subdomain and hosting them there.
Add Expires headers
- Web pages are becoming increasingly complex with more scripts, style sheets, images, and Flash on them. A first-time visit to a page may require several HTTP requests to load all the components. By using Expires headers these components become cacheable, which avoids unnecessary HTTP requests on subsequent page views. Expires headers are most often associated with images, but they can and should be used on all page components including scripts, style sheets, and Flash.
Make fewer HTTP requests
- Decreasing the number of components on a page reduces the number of HTTP requests required to render the page, resulting in faster page loads. Some ways to reduce the number of components include: combine files, combine multiple scripts into one script, combine multiple CSS files into one style sheet, and use CSS Sprites and image maps.
Reduce DNS lookups
- The Domain Name System (DNS) maps hostnames to IP addresses, just like phonebooks map people’s names to their phone numbers. When you type URL www.yahoo.com into the browser, the browser contacts a DNS resolver that returns the server’s IP address. DNS has a cost; typically it takes 20 to 120 milliseconds for it to look up the IP address for a hostname. The browser cannot download anything from the host until the lookup completes.
Avoid HTTP 404 (Not Found) error
- A favicon is an icon associated with a web page; this icon resides in the favicon.ico file in the server’s root. Since the browser requests this file, it needs to be present; if it is missing, the browser returns a 404 error (see “Avoid HTTP 404 (Not Found) error” above). Since favicon.ico resides in the server’s root, each time the browser requests this file, the cookies for the server’s root are sent. Making the favicon small and reducing the cookie size for the server’s root cookies improves performance for retrieving the favicon. Making favicon.ico cacheable avoids frequent requests for it.
Avoid empty src or href
- You may expect a browser to do nothing when it encounters an empty image src. However, it is not the case in most browsers. IE makes a request to the directory in which the page is located; Safari, Chrome, Firefox 3 and earlier make a request to the actual page itself. This behavior could possibly corrupt user data, waste server computing cycles generating a page that will never be viewed, and in the worst case, cripple your servers by sending a large amount of unexpected traffic.
Put JavaScript at bottom
- JavaScript scripts block parallel downloads; that is, when a script is downloading, the browser will not start any other downloads. To help the page load faster, move scripts to the bottom of the page if they are deferrable.
