As a web dev I always have this in mind but the challenge is convincing your client who wants a video background. Maybe we need a media query that detects internet speed.
Even then, it's fairly easy to load the video asynchronously and as part of the last assets, make its intro blend in to a single-color background, voila, problem "solved".
I find video backgrounds ridiculous most of the time (though they can be done really well), but that's not the real problem here - the real problem is including 200-800kb javascript code that does nothing but track your user, and often enough doesn't do it for you! (Hi Facebook!)
The real problem is using massive js frameworks for the sake of adding dynamic functionality to your site that, often enough, isn't actually worth it.
The real problem is that very often, these "features" are only as necessary as the marketing team says they are... the people who have the ability to ask "why?" and the ability to understand "why not" don't have the voice (or guts...) to do so.
"Massive" JS frameworks aren't the issue, not by themselves. AngularJS, for instance, is only 39.5kb. Bloat sneaks into web applications in other ways, but merely bringing in a framework isn't enough to add a noticeable load on a web page.
I would love this. My company just updated their public-facing website and it has a 28MB 1080p video playing in the page header. The site was outsourced, but I work as a web developer for some of our products and was asked by the marketing team to give the new site a look over.
The marketing team was surprised to learn how big the file was since the outsourced team "ensured" them that the new website would be lightweight. Oh well... Now the video has been reduced to a 16MB at 720p, which is still ludicrous to me, but they just really like having the video on the home page more than they want a truly lightweight page.
And to be fair, the video does not load when viewed on a mobile device, so at least there's that.
