think there is no faster solution when there are so many rowsthat are on your server 157 rows per second
check user id if mysql num rows = 0 than delete the downloads and also check the item_id
there was also a similar question about the performance of myswl num rows
MySQL: Fastest way to count number of rows
edit: think the best is to creatse some triggers so the database server does the job for you
currently i would use a cronjob for the first time