Sorry, I wasn’t checking forum for a while. Well, it all depends on your situation, but maybe you could create an UObject that would be a wrapper to an array of ints and also has an int as sum of all elements. And whenever you change elements in that object’s array you could trigger recalculate of the sum, so then you need to calculate everything just once.
Edit: In C++ you could do that with struct as well, as structs can have functions in C++. Unfortunately Blueprints don’t support struct’s functions.