The Domain Mapping plugin accounts for 60% of the plugin load time according to P3, at about .6 seconds. Since this is more than 6x slower than than the next slowest plugin, I figured I should look into it.
This was on a sub-site with NO domain assigned.
This was as an anon user. When I tested with a user that was only assigned to this account as admin, the time went up to .9 seconds.
Cross-domain login is turned-off.