Loading video content in "theory"
In some lessons, for example in animation lessons, videos are used instead of pictures and they all start at the same time, which makes weak devices slow down.
I suggest loading and starting the video only as soon as it becomes visible, and when the video disappears from view, immediately stop it.
Better yet, add the ability for the user to decide when to start and stop the video.
And add in the settings a choice of how the video will work.