Set Actor Location slows down ?

I’ve seen a couple of posts mentioning “Pure” functions, still not entirely sure what that means. Also the difference between a “Copy” and “Reference”. Thing is, when I use Set Actor Location especially following the mouse location after several moves it starts to slow down quite a lot ?

Edit: Just found, I think this mostly explains the Pure function. I had in mind something else but must have been due to the description. The “WFT is ?” series are really good.