UPDATE  post
SET     Votecount = Votecount - 1
WHERE   post_id = ?
        AND post_id NOT IN
        (
        SELECT  post_id
        FROM    Votes_table
        WHERE   user_id = ?
        )

你仍然需要运行第三个.

确保在voting_table(user_id,post_id)和post(post_id)上有唯一索引或Primary KEY.

最有可能的是,您当前的查询缓慢是由于缺少这些索引引起的.