Comment by bluGill

Human factors has a long history of studying this. I'm 30 years out of school and wouldn't know where to find my notes (and thus references) , but there are places where users will notice 5ms. There are other places where seconds are not noticed.

The web forced people to get used to very long latency and so fail no longer comment on 10+ seconds but the old studies prove they notice them and shorter waits would drive better "feelings". Back in the old days (of 25mhz CPUs!) we had numbers of how long your application could take to do various things before users would become dissatisfied. Most of the time dissatisfied is not something they would blame on the latency even though the lab test proved that was the issue, instead it was a general 'feeling' they would be unable to explain.

There are many many different factors that UI studies used to measure. Lag in the mouse was a big problem, not just the point movement either: if the user clicks you have only so long before it must be obvious that the application saw a click (My laptop fails at this when I click on a link), but didn't have to bring up the respond nearly as fast so long as users could tell it was processing.