This started happening on one of my older computers recently and I had to actually disable hardware acceleration to get the audio to stop being out of sync. The tradeoff for that has been noticeable frame dropping. I even added an extension to force h264 but there's no difference.

It can play video no problem on other sites or in VLC. It's just YouTube and just on that PC...That PC can run Doom Eternal. Video content shouldn't be a problem.

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.

Given their actions the last two years or so unfortunately the default assumption is (reasonably) “probably something intentional to get people to stop doing something they don’t want, like using ad blockers.”