My research:
The only related question I could find is specific to JWPlayer. My question isn't specific to JWPlayer:
Optimal file size for a html5 background video stream with JWPlayer?
Search engine results don't seem to yield a guideline either.
My question:
Before answering that there is no optimal and the world is far too beautiful and replete in variation for one to fathom an answer, bear in mind that I'm basing my question on the average or most common internet connections.
To put it another way, if I wanted to reach the majority of internet users, without experiencing significant load/performance issues, what should my maximum file size limitation be?
According to the Akami 2014 internet speed ratings, the world average internet speed is 3.9Mbps (That's about 500KB per second or half a megabyte). However, I would strongly suggest not aiming for average, but aiming below average for optimization so that even more people have a pleasant experience on your site. I would suggest aiming to optimize for speeds around 1.5Mbps (about 185KB per second). That means if you want your page to load in a reasonable 10 seconds on a slow 1.5Mb connection, uncached (the first time someone visits your site, it will speed up as images are cached) your total page should be less then 2MB. So you have to ask yourself how much of that 2MB are you willing to use on a background image? I would suggest aiming for around 150-300Kb for a background image. Use a program like Photoshop and the "Publish to Web" feature to play with your background images settings and file types to change it's size to what you want. I suggest avoiding Jpeg and aiming for PNG.
来源：https://stackoverflow.com/questions/31625629/whats-the-optimal-file-size-for-a-looping-html5-background