In my opinion, loading is the main SEO factor, period!
Because it influences some of other parameters, like bouncerate. For example; somebody enters the website and waits for 15s for the page to load.. that is awful..
You might think that websites loading speed trend is decreasing, but NEAH.. As CMS's like Wordpress, and e-commerce like Magento that are more and more customisabled ..
As you wait for first page to load, you tend to leave the website, hence, a bigger bouncerate. Google sees that and do ranking penalties to your business/website. Usualy you cannot have a low bouncerate with a big load.. that's a thing I never saw..