Google Search and Microsoft's Bing teams, Jake Brutlag and Eric Schurman respectively, presented the results of user performance tests at today's Velocity Conference. The talk was entitled “The User and Business Impact of Server Delays, Additional Bytes, and HTTP Chunking in Web Search”. These are long-term tests were designed to see what aspects of performance are most important. To know how to improve their sites both Bing and Google need to know what tweaks to page load perceptions and realities help or hurt the user experience. This is one of the first performance tests that has actual data (and is not strictly anecdotal). The numbers may seem small, but they if you are dealing in millions/billions they add up quickly.
More info: O'Reilly Radar