The slow query log can be a great tool for diagnosing and fixing slow queries. It can also slow the server to a crawl if you are doing bulk inserting. MySQL isn’t afraid to write any query to the slow query log. Even ones that are hundreds of megs. If you are using the multiple-row insert syntax and a query takes longer than long_query_time when it completes mysql will write the entire query out to the slow query log effectively doubling the ammount of data written to the disk.