I have approximately the same Macbook (but from 2015), one issue i have is playing 4K .webm videos on Youtube. It doesn't matter which browser, it even happens when i download the video and play it in another player. My CPU will spike and the fans start blasting.
4K videos in mkv/H.264 format i can play fine though.
The issue here is the hardware decoders capability;
Depending on which browser you use, and whether your Macbook is connected to power, websites will send you a different video/codec/size.
It works fine in another browser. So I don't think it is a hardware issue? Is there a situation where Chrome/Vivaldi would be able to use hardware the FF wouldn't? And 90% of my time is spent on power (top left usb-c port).
4K videos in mkv/H.264 format i can play fine though.
The issue here is the hardware decoders capability;
Depending on which browser you use, and whether your Macbook is connected to power, websites will send you a different video/codec/size.
Here's a random post that sort of describes this: https://apple.stackexchange.com/a/409764