Sorry I started to reply and then suddenly realized I had to leave so I kinda shortened it a lot.
Just so you know, a lot of this may be unnecessary, unrelated and/or unimportant info.
I noticed if I turn off Avast when I have a video already open, the video instantly appears to be already loaded, though I've only tried this twice, and once the video froze.
Taking a moment to compare load times of different videos with avast on and off.
This may take a few tries to determine if there is a relation, as sometimes Youtube videos just load really slow, followed by other videos loading really fast.
Will post again later.