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Web Performance Statistic of the Week 12/6
Performance issues on websites during the process of completing a transaction will cause thirty-seven to forty-nine percent (37-49%) of users to abandon the site or switch to a competitor. The lost revenue from this experience is bad, but the worst part may be that 77% will make sure other people know about the problems. You will never know how much money is lost and how much damage was done to your brand reputation.
From Velocity 2010 presentation: Metrics 101: What to Measure on Your Website