Following mysql query is taking too much time for excecution

I have a MySQL table which is described in the following question.Counting number of ids present in one table which is not presented on another for a particular userThe query which I am using is as follows.

SELECT count(*) coincidencesFROM   table1WHERE  user_id = 12AND    NOT EXISTS (SELECT 1                   FROM   table2                   WHERE  tableid = table1.id                   AND    user_id = table1.user_id)

This query is taking too much time for considerably bigger table how can I improve the performance of this particular query.