If the video is encoded using a codec your hardware doesn't handle, it would be left up to the CPU to decode. Av1 can slow everything down to a crawl over CPU. You'd think the browser would be smart about the stream selection though.
I run an extension that allows to automatically request h.264 streams from YouTube even when av1 is also available. Saves a lot of CPU, at the cost of some bandwidth.
I recall h264ify but not sure about it