[mxmilkb]

I tried this because FF doesn't do hardware video decoding yet, and, on an older laptop, FF was taking up a whole core for watching YouTube vs. around 30% in mpv.

Unfortunately every "open with.." solution I tried though took 10-15 seconds to open the video in mpv, which turned out to be even more frustrating than the problem.

[catalogia]

I've experienced what you describe. I think the problem is most severe when you're on a high-latency connection, such as a VPN to an endpoint far away, because there are a few round trips in the process with youtube-dl. I don't really have a solution to that, but using --force-window=immediate can help a little (it will make a blank mpv window appear immediately, before the video is ready, so at least you aren't left wondering whether you misclicked prior to the video starting.)