Well, if you ask an SEO geek like me:
As long as the video doesn’t auto load, it should be made available both on desktop and mobile and its size/download time will not have any effect on the page load time of the page where it sits.
- Always include the video for mobile users, at least as an option to view. Just having a still image isn’t going to do anything for mobile users, and their numbers continue to grow by leaps and bounds.
- I would consider always using the low-res unless it’s really bad or unless detail is crucial. If you want, you could include an option to run either the high-res with a note that it takes longer to load the high-res version. (but nearly all videos I’ve seen on mobile stream anyway).
- As far as ideal time length goes, ask 100 people and get 100 answers. Of course, the longer the video the larger the file. I know if I see something more than 5 or 6 minutes, I often won’t click on it or, if I do, drop out before it’s over.