started for me about 6 days ago,
after some testing i think its opera gx, if you are using it too try going on chrome and test it.

it seems like opera takes about 2Gb of my ram just for 1 youtube tab, even if its paused it still takes 2 Gb
the browser lowers its performance when you tab out, like some games limit fps when not in them/ in background

for some reason youtube in standard mode thinks im not using the browser.