Long load time caused by dm-sso-endpoint/1490015210/?dm_action=domainmap-check-login...

Hi There,
I have a multisite and use domain mapping so users can use their own domain names instead of my subdomain.
A few users have complained about long load times for their sites - I'm looking into this today and it seems like it's releated to domain mapping. When I run a speed test, this is the resource that takes a long time to load (it has a long wait time):

http://skizzar.com/dm-sso-endpoint/1490015210/?dm_action=domainmap-check-login-status&domain=sortrehearsalrooms.com

Is there a way to reduce the load time - I'm not too sure what this resource even does?