It's lag. Lag that happens when the fight music starts.
Some will tell you that it's solely on your end. But since the lag situation is pretty consistent from a wide number of people presumably living in different zone, something is coming down the pipe at that time that's slowing your game down. This seems to have only started recently as well. I don't know what it is since everything seems to be the same as before in the mission. Perhaps the music is being sent differently?
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