Content Delivery Networks- Web performance can be done by using CDN.
Basically, they have multiple servers across the world and when a request comes for
yourcompany.com for example, the CDN "owns" the DNS for that name and static .html, JavaScript files, and image files can be served from their edge server. If their edge server doesns't contain your .html, image, or JavaScript, their edge server sends a request back to your web server transparently to another domain (another.yourcompany.com) on the internet to retrieve the files.
The files then get sent back to their edge server and stored there and then sent to the client. If your file is at their edge server, the client machine (requestor) gets the files from their edge server. Thus if your web server is sitting in San Francisco and someone from London wants to visit your web site, your site content can be served from a CDN server in London instead of San Francisco, reducing response time.
Obviously, the time it takes for data to transfer within London is a lot less than from London to San Francisco. CDNs can also provide live streaming functionality - serving bandwidth intensive media from their content servers... something that can be costly if you are a multimedia-rich company. Watch out for testing results CDN companies give that are irrelevant to your site. Any test results of downloading large single 300kb .gif files are not indicative of how well a CDN's service is appropriate to you.