Are you running that in the weapon blueprint itself? So is the weapon rotating itself or do you have a separate blueprint that holds references to all your weapons and rotates them. How often are you calling that function? On tick?
If that is located in the weapon blueprint, then you actually have 20 turrets and they each are told to create an array with 20 actors to rotate. If that is the case you have 20 arrays with 20 actors each being told to rotate which depending on how often this function is called could be an issue.