The main reason I believe this is occurring is because hardware accelerated video decode is not working correctly. Turning hardware acceleration off and then on again seems to fix this for me. I can tell if it's working by how long youtube videos take to load or how high my fans spin (on a simple 1080p youtube video...)
Another step I used was looking at Opera://GPU and seeing the Hardware Video Decode status.
For me it said:
Hardware Protected Video Decode: Unavailable
Metal: Disabled (I am on Mac)
I changed the Flag: Override software rendering list and now it says my hardware protected video decode is enabled.
This is occurring on other chromium based browsers as well (Chrome & Brave)
I am on MacOS 10.15.7 with Macbook Pro 13 inch 2017