I think I have average specs, 16GB of RAM and a 3070, but I can't get the videos to play without stuttering and slowing down so much, that they take 10x as long as they should, either. The first second plays flawlessly, as it is already pre-loaded, but the rest takes ages only to move forward stutteringly by half a second in slow motion.
Load doesn't seem to be the issue, it's in the single digit percentages for both CPU and RAM, and my SSD is barely even doing anything.
Seems like a software problem to me, perhaps only affecting some setups (I tested this on windows 11). Maybe windows defender acting up (though I should have been able to see that in task manager), or just the game loading the videos and audio files very inefficiently, garbage collection acting up, who knows.
It could also be a plugin issue, but it will be hard to find the offender if you can't reproduce it yourself.
If you want to change to another engine, I would also advise against html, I think sugarcube and similar are good engines, but if you don't have much programming experience, or ideally have experience as a web-dev, it'll be hard to program good mini games or design nice pages. Html is also more suited for text-heavy games, RenPy would suit your visual style better.