improve 'where not exists' query..

I'm wondering if there's an ideal way to improve the efficiency of this
query:

SELECT i.id <http://i.id> FROM items i
WHERE (NOT (EXISTS (SELECT c.id <http://c.id>
FROM contacts c WHERE (c.id <http://c.id> = i.id <http://i.id>))));

It takes a while to execute, clearly. Thank you,