Page Response time Perception
The visibility of a website or a web page in a search engine's unpaid results depends on its loading speed. Since 2010, Google announced that Webmasters should optimize their web pages loading speed because it's going to be a ranking criteria: Page speed and Search Engine Ranking. Matt Cutts predicted this change on his blog.
Moreover, global website performance is also important for your visitors. Most people dislike when a website is stuck loading the page, and leave without coming back. With internet being faster and faster, people become more impatient and refuse to wait.
With increasing network speed, both on mobile and desktop, users are used to quick page loading. This would have been a completely different story when users were connecting using 56k. Did you know that most people leave a website after 2 seconds of waiting?