Youtube lagging with dropped frames when interface is visible

On two different computers, starting 2-3 days ago, Youtube video will slow down, and drop frames, then catch up to the audio after a few seconds, over and over.
This only happens when the mouse is over the video window, causing the Youtube interface to appear. Runs fine otherwise.
It is not a buffering issue as I see the buffer indicator in the interface far ahead of where I am in the timeline.
Is not happening on Edge or Chrome.
Tried disabling all extensions.
All flags are default.
It may only be happening on 60fps videos? (doesnt happen on every video)

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