If a page takes too long to load, after how long – generally – will a user give up and re-load or go elsewhere? What studies have been done that measured page load time and user satisfaction or abandonment?
Jakob Nielsen has an excellent article, Website Response Times (June 21, 2010) in which he explains why website response times still matter.
He cites his Response Times: The 3 Important Limits as coming from accepted research as old as 40 years.
Short answer: roughly 3 seconds for more than half of users.
In Web Performance Today's Cheat Sheet: Everything you wanted to know about web performance but were afraid to ask, the first point addressed is the effect of page-loads.
In 2006, the average online shopper expected a web page to load in 4 seconds. Today, that same shopper expects your page to load in 2 seconds or less. [Source: Forrester Consulting]
Other information in the article (all cited as in the quote) includes the effect of millisecond improvements in page-load times on page-view times, the effect of page-load times on time spent viewing advertisements, and the various effects of page-load times on revenue for e-commerce sites.