Will the video background slow down my site?
The video background do not interfere with the page loading time, as the video loading is triggered after the completion of the page loading. See, for example, our demo at https://www.codeblackbelt.com/shopify-js-apps/demo/video-background-featured.html
However, the video background will affect the page performance, as video playing consume CPU and bandwidth resources. For that reason, we do not recommend to set many background videos in the same page or to select huge areas for the video background.