The speed and size of their Web servers. The more CPU power dedicated to handling server requests, the better. High end servers running UNIX are usually the performance leaders. This configuration is extremely stable, and reliably handles millions of hits a day while maintaining high performance. Multiple diverse connections.
Internet traffic can take many different paths to reach a given destination.
Some paths are faster or more direct than others. Peering points are where traffic from the large backbone providers get transferred from one network to another. Like an interchange between large interstate highways, traffic often slows to a crawl. A big problem with the Internet today is the saturation of these peering points. If your data has more choices and can avoid these points, it will arrive more quickly and more reliably.