Google research: “the cost of slower performance increases over time and persists“. In other words, if your website is a little slower, users will use it less (we knew that), but they’ll also use it less and less over time, and when it speeds up again, they’ll still use it less than before the slowdown.

And an interesting insight into how Google thinks about speed: “Because the cost of slower performance increases over time and persists, we encourage site designers to think twice about adding a feature that hurts performance if the benefit of the feature is unproven.”