Marissa Mayer explained yesterday at Web 2.0 Summit that speed is one of the key issues for a web application. She said that the major reason why Google shows 10 search results by default is that the page loads faster (on average, in 0.4 seconds).
When launched, Google Maps was pretty slow, but a speed improvement brought a lot more traffic. "When the Google Maps home page was put on a diet, shrunk from 100K to about 70K to 80K, traffic was up 10 percent the first week and in the following three weeks, 25 percent more."
Also the upload process in Google Video took a day or two, so people couldn't see their video online instantly. Marissa says that's one reason why YouTube had more success than Google Video.
Instant gratification plays a big role in a great user experience. Just imagine a search result page loading in 20 seconds. How often would you use that search engine, even if the results are great?