Many youtube videos consist of silent video streams combined with separate audio streams. Youtube-dl can list them. Often, the lower-quality and some medium-quality video stream will also contain audio, but the higher-quality videos are often silent with a second audio stream mixed in.

I can imagine that your older computer might have native support for decoding the high-res h264 video, but not for the high-quality OPUS audio stream, causing them to decode at different speeds.

I don't think there's a solution to that problem that doesn't involve running a dedicated video player such as VLC.

That said, I've also heard a lot of complaints from people still using Adblock Plus. I don't know what rules ABP has implemented, but they seem to affect Youtube performance much worse than uBlock whenever Youtube changes something.