Try the following for all user related changes i.e. add /remove /modify privileges for your user
Those changes are instant. However, if you reboot the server or restart the mysql server, changes will be lost. So you need to edit the /etc/my.cnf. Add slow query related changes and reload the mysql server using service command. Example my.cnf:
I want to add slow_query log in mysql> terminal
how can I affect that change without restart mysqld daemon
log_slow_queries = /var/log/mysql/mysql-slow.log
long_query_time = 2
The location of my.cnf and service name may change according to the Linux distro.
service mysql reload