Why site speed has been introduced in Google Analytics interface?

Why site speed has been introduced in Google Analytics interface? Will that be a metric for ranking?

Yes and it has been for quite some time.

Google want to provide a quality service to their users so as well as looking at all the factors that they consider to represent good quality and relevance, they also want pages that load fast.

It’s very unlikely that a difference of a few milliseconds will make any difference - what they are trying to do is to weed out sites that load really slowly.

Google may want to give equal weightage to the internal optimization of the website as is the case with external link building.

Site speed helps website owners to identify slow loading pages so that they improve their loading time and get good results. Site Speed report also displays which are the slowest loading pages, whether visitors from different geographical regions are experiencing different loading speeds, and whether a site loads faster or slower for different browsers.
Slow loading time directly effects your traffic.

Google is obsessed with page speed and if you do not believe this then look at the code for google.com. There is no closing body or html tags because they found that the removal of these tags sped up their site by a few milliseconds and decreased bandwidth which makes a difference when you are serving up billions of pages a day.

I believe the inclusion of page speed metrics in GA is just another indication of the importance they place on site speed and I think it may become an even more important ranking signal as time goes by, so personally I would begin making changes now to speed up your site.

As on-page is the important task. This task give detailed information about site, which most of the search engine use to search for the best relevant results for their user. And site load time is on of the factor from this.