google on web master things
Google did a experiment on how speed effect users. Interesting reading. Speed Matters (2009-06-23) by Jake Brutlag. Source
A more informative article with clear summary is here: Bing and Google Agree: Slow Pages Lose Users (2009-06-23) by Brady Forrest. Source
Of course we all know that we don't like slow pages, but the interesting thing is that even a 0.4 sec delay has measurable impact.
Another, about minor and major experiments google is doing: Search experiments, large and small (2008-08-26) by Ben Gomes. Source
More readings: http://googleblog.blogspot.com/2008/08/search-quality-continued.html. A interesting thing on the site is this link: http://www.google.com/jobs/britney.html, showing the few hundred variation of spellings people used to search Britney Spears. From this, we can infer how much trouble they take just for the spell correction feature, and further, how extremely complex the search technology actually is, beneath the extremely simple interface.