According to a study done by OnlineGraduatePrograms.com in 2012, one out of four people abandon a website if it loads more than four seconds. Amazon, in another research, calculated that a page load slowdown of just one second could cost them to lose $1.6 billion in sales every year.
These are just two studies about page speed, but if you do a quick Google search, you can find thousands of similar studies showing how important speed optimization is to a certain website.