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9. Use a Content Delivery Network (CDN)
Your site’s speed is greatly affected by where the user’s location is, relative to your web server. The farther away they are, the more distance the data being transmitted has to travel. Having your content cached across multiple, strategically placed geographical locations helps take care of this problem.
A CDN will often make your operating cost a little higher, but you definitely gain a speed bonus. Check out MaxCDN or Amazon Simple Storage Service (Amazon S3).
10. Optimize Web Caching
Along with using caching systems, you should create websites that utilize web caching as much as possible. Web caching is when files are cached by the web browser for later use. Things that browsers can cache include CSS files, JavaScript files, and images.
Aside from the basics, such as putting CSS and JavaScript code that are used in multiple pages in external files, there are many ways to make sure that you are caching your files in the most efficient way possible.
For example, you can set HTTP response headers such as Expires and Last-Modified to reduce the need of re-downloading certain files when the user comes back to your site. To learn more, read about caching in HTTP and leveraging browser caching.
To set up HTTP Expires headers in Apache, read this tutorial on adding future expires headers.
Other mentions
Avoid Resizing Images in HTML (using HTML’s width and height attributes), for the sake of smaller file size.
Optimize Image Sizes by Using the Correct File Format. Eg. JPG format often displays photographic images at smaller file sizes than PNG.
Optimize the Way You Write Code. For example, instead of using (h1)(em)Your heading(em)(h1), you can easily use CSS to make your headings italics.
Writing code efficiently not only reduces file sizes of your HTML and CSS documents, but also makes it easier to maintain.
Ref: http://sixrevisions.com/web-development/site-speed-performance/