CPU overheating since prepatch

When I am fighting the event bosses with hundred of other players my CPU overheats, causing my computer to shut down.

The first time it happened I thought it was my graphics card so I turned all the graphics setting to their minimum setting, but my PC kept shutting down when there was a large group of other players.

The last time I went to fight a boss I watched the CPU temp and activity and noticed that the game seems to using only one CPU core, and as soon as everyone starts fighting that one cores temp starts steadily rising.

Why isn’t the game using more than one core?