Maybe it's because they found what they wanted, not because of load time
The article states:
After increasing the number of first page results to 30 for a group of guinea pigs, Google watched as the number of searches dropped by 20 per cent. "It turns out that it takes us longer to produce 30 results per page," Mayer explained. "And that latency drove the decline."
Um, I hope she has better evidence... I'd say people just found what they wanted in fewer page loads. The extra load time for 30 results versus 10 is negligible.