[MM3-users] Domain Resolving Issue

I am running a Mailman 3 server with about 5 list domains configured on it. Most of them resolve quickly both to Postorius and HyperKitty. However there is one domain that resolves very slowly. Once it does, access to other pages/tasks go quickly. If you don't do anything with that domain in the browser after about 5 minutes, it takes its time again to resolve. What could be causing this? The webserver is Nginx. Not sure if this is a Mailman 3 issue or not.

Brian