Even small changes in response times can have significant effects. Google found that moving from a 10-result page loading in 0.4 seconds to a 30-result page loading in 0.9 seconds decreased traffic and ad revenues by 20%. When the home page of Google Maps was reduced from 100KB to 70-80KB, traffic went up 10% in the first week, and an additional 25% in the following three weeks. Tests at Amazon revealed similar results: every 100 ms increase in load time of Amazon.com decreased sales by 1%.
Citations omitted. Via A List Apart.