MySQL Slow Query Log File

MySQL has a feature that allows you to log slow running queries to a file. To enable you just need to add some lines to your my.cnf file, and restart. Add the following:

log-slow-queries = /var/log/mysql/mysql-slow.log
long_query_time = 1

The above will log queries taking longer than one second to the specified log file.

You can read more in the MySQL Reference Manual