From the chart you may clearly notice that the website conversion rate goes down proportionally with the increase of a web page response time: more seconds mean fewer conversions. Reasons are quite simple to understand and are represented by the overload of contents offered in the internet: if the user experince is bad, the plenty of similar resources, if not identical, moves users to somewhere else (if I can not buy it here, I will buy it on a faster website).
So it’s clear that the site speed of an e-commerce seems to be one of the main reason of user dissatisfaction, other than, in “more serious” cases, of website abandon or not finalisation of purchase.