PageSpeed is the amount of time it took web server to respond to the requests of the browser and how long it takes for it to serve all page Resource(Images, Audio, Videos, Flash Animations, e.t.c.), Content (HTML, CSS and Texts) and Meta Information (HTTP info i.e Author, Content-Type, e.t.c) to be fully downloaded and rendered by the web browser. Modern browsers have been optimized to handle page loading efficiently with time, They do have their own drawbacks but not as expensive as a slow webpage.

A slowly loading webpage is not good for user experience, most users just click away or close the tab before the loading completes, These can make any potential buyer leave before the get to see any product on the website. PageSpeed improvement is a way of gaining more power above market competitors, People can browse through contents faster, make payments and review / comments. It encourage them so browse around more categories and purchase more items.
source : http://www.codingsavvy.com/2017/06/killer-tips-to-improve-website.html