352205 Mysql update

Hello, Regular mysql update query works very slow with 1.5 million records. I am using this query: UPDATE posts set post_userid='$var1',post_username='$var2',post_message='$var3',post_date='$var4',post_dateunix='$var5',fixed='1' where post_id='$varr' But it takes ~6-7 minutes to complete this query. I need someone who is expert in mysql and offer me fastest solution. PS: I already optimized table.