Cocoa @sum array operator too slow - alternatives?

I have a textbox value associated with a key path using @sum. When the array filter is filtered, the text box updates as expected, but extremely slowly, lagging behind the UI or a straight beach bed after each keystroke (less than the number of items in the ordered objects decreases). There is no lag if I don't use @sum; I have narrowed it down.

Is there a faster way to do the same?