If you're running Wordpress on an IIS server then there are some great DNS performance gains to be had by changing the DB_Host value in your WP-Config file.
WARNING - All changes of this nature should be carried out on a test site first.
Providing that you have a standard Wordpress installation, with your MySQL database running on the same server, then making this change should not cause any problems.
The DNS lookup performance benefits are huge. Having tested sites using Pingdom and Google Page Speed insights, the DNS lookup time reduces from approximately 300ms to between 0 and 5ms!
By default the WP-config file uses 'localhost' for the DB_HOST value. With IIS the server has to resolve localhost everytime a page is requested. On a default install your localhost IP address will be 127.0.0.1
Therefore by changing define( 'DB_HOST', 'localhost' ); to define( 'DB_HOST', '127.0.0.1' ); IIS does not need to perform a DNS lookup.