N1QL DELETE statement getting slow

DELETE FROM content 
WHERE _type="UserAsset" AND _id="887a35e9-81cc-40b3-bb0d-657a64caed81"

The above query is taking arround 20 seconds to execute. How to index for the above query so that it will execute in millisecs or I have to write the query in a diffrent way?

CREATE INDEX ix1 ON content(_type, _id);