Does theme loading speed effect SEO?!

[macaulead]

If a site's theme is heavy and takes a lot of time to load. Does it mean that it effect the site's SEO negatively?

Many webmasters say that it has effect. But yet I see a lot of site who takes a lot of time to load still manages to rank very high.

Page speed is also important to user experience. Pages with a longer load time tend to have higher bounce rates and lower average time on page.

True. But in many cases page's loading speed isn't much depended on the theme. Themes usually does load fine. If it hasn't been bombarded with media and high resolution images.