Youtube has been turning on AV1 for 1080p content for me. My phone is the only device with an AV1 hardware decoder. The impact on battery life and CPU usage has been extreme.
You can tell Youtube to prefer AV1 only for low-quality videos (https://www.youtube.com/account_playback) or you can install an extension that will force h264 playback where supported.
Other playback features such as ambient mode and volume equalisation can also impact performance, though that kind of depends on how fast your web browser is at executing Javascript, and to a much smaller extent.
The substantial bandwidth savings are here to stay, though, so in time I think Youtube will move to AV1 more often.