It was being recalculated repeatedly.
This memoizes it, although it still re-calculates occasionally despite none of the source arrays changed. I think it is due to the state change in the Preference Sync.
Note: input selectors to `createSelector` needs to be extractions-only (i.e. must not have transformations). I think most of our `makeSelect*` selectors violate this and broke memoization.
a8149fe9bb