Content Delivery Networks
A content delivery network (CDN) is a server that you store your images and other static content on, and it serves it up from one of several data centers. The gain is twofold - content loading from your server is generally loaded sequentially (a simplification) and data loaded from multiple sources is loaded in parallel. The content from the CDN is loaded from a different domain than the main content, and so can be loaded in parallel. In addition, the data from the CDN is served from the location closest to the visitor, saving time.
A Switch to Rackspace
I do not use a content delivery network (CDN) for this web site. I use them at work, and since Google started complaining about load time on one of my sites, I decided to give it a try on that site. I started with cdn.net. I used them because they are inexpensive. I gave them $10 6 months ago and I have $8.20 left now. I changed to Rackspace, also inexpensive, but in a different class. cdn.net is a reseller for another CDN outfit. Rackspace uses their own datacenters, and also colocates with others.
Why the change? Google started complaining again. At first I just thought it was a passing thing, but as time passed it got progressively worse. Eventually, the pages would spin waiting for one image or another to load. The worst test case was 26 seconds. Not acceptable. 2.6 seconds is not acceptable.
I looked for another CDN, finding others that are resellers of the one in question. Eventually, deciding I was going to have to bite the bullet and pay for the CDN, I looked at Rackspace. They charge for the storage and the bandwidth. $0.10/GB/Month for storage, and $0.12 per GB bandwidth. I have a few tens of MB of data and use about 8GB/Month bandwidth. It costs about nothing for storage and $1.00/Month for bandwidth.
It is a little more expensive, but now my average load time for one of the cached resources is around 35mS, rather than several seconds. Google no longer complains, visitors no longer leave from boredom, and I don't sit here and fret about it. It's a win for everyone.