I just realize that Youtube on Opera is really really really slow responding when they are running in 1080p60. If you open 4-5 Youtube tabs or more, it gets worse.
At first time, I thought it's because my PC is too weak (I'm using Dell M4800 - i7-4810MQ & Quadro K2100M, I know it's very old model but believe me it's not that bad) but when I open Chrome and start to running exactly like Opera (the same amount of tabs, the same URLs). Then Chrome runs normal.
So I think that's because Opera is doing something wrong. Please fix it in the next updating.