This is an interesting topic of conversation. There are numerous studies showing an effect of loading speed on SEO and rankings, and a couple of comments from Google sources directly that there is an implication.
Exact numbers are not readily available, although there are a lot of speculative numbers being thrown around.
Let's start by putting everyone at ease. Google official blog recently announced:
“While site speed is a new signal, it doesn't carry as much weight as the relevance of a page. Currently, fewer than 1% of search queries are affected by the site speed signal”
In other words, don't stress over website speed too much.
However, website speed has tremendous implications on conversion rates.