Write in help text that users have to be aware of that video – spec. big ones – can put down the performance time at the specific page
No, because the problem is not due to large videos. I said:
As for the videos, the one I can see displayed quite fast.
I will do more tests, but I think you have a problem with the thumbnail for that large video. Is it possible you have a copy of the video accidentally stored in the folder /data/media/thumbs/video ? That would give an explanation, because the page would try to load the very large thumbnail from there.