The domain mapping plugin is slowing down the admin area of my site, especially the Plugins page at the Network level. It causes it to time out because it takes over 100 seconds to load. Occasionally it will load, but usually not. I contacted my hosting provider, WP Engine and they tracked down the issue to the Domain Mapping plugin.
Here's what they said:
The domain-mapping plugin is doing around 396 queries to pull the siteurl of each subsite as well as all the plugins that are installed.
They suggested I contact you to see if there is a way to defer that DB query from loading on other wp-admin pages.
Here is the query:
SELECT option_value FROM wp_options WHERE option_name = 'siteurl'
coming from this file: wp-content/plugins/domain-mapping/classes/Domainmap/Module/Mapping.php on line 592