When using a video as a background, does linking to youtube use less data than uploading the video itself?

I've been having numerous problems with SS and they're telling me it's bc the homepage of the website uses 15 MB of data rather than the recommended 5 mb. The biggest thing I have on there is a 12 second looping video- it's 12mb. I just readded it as a Youtube link instead. Will this lower the data usage of the site?

Yes, that should improve the delivery, YouTube has quite a serious compression algorithm, and the delivery method via YouTube does generally improve the loading. What version of SQSP are you using?