A common data point I see brought in web performance discussions is a reference to a test Google ran to test the impact of slower search results on the number of searches a user performs. I found the original blog post at Speed Matters:
Our experiments demonstrate that slowing down the search results page by 100 to 400 milliseconds has a measurable impact on the number of searches per user of -0.2% to -0.6% (averaged over four or six weeks depending on the experiment).
The impact extends beyond the initial test period:
Users exposed to the 400 ms delay for six weeks did 0.21% fewer searches on average during the five week period after we stopped injecting the delay.
If you are going to reference this test and the corresponding data, please link back to original Google blog post. Hopefully that will save others the time of having to hunt down the original information.