I choose a music on YouTube, so I can relax during my gameplay, but then I open the game and it doesn't want to work. The home screen started to lag very badly, any chosen buttons in menu didn't work. Music also started to lag, though it was fully loaded. Tried it few times with different types of websites open, but every time the reaction was the same. I manage to turn off Minecraft via Task Manager.

I did the same (playing MC with music from YT in background) on earlier versions (like 1.14, 1.15). Last time I did it was 20w18a and it worked properly.