What is a Synchronous Registry?
A Synchronous Registry is a domain name registry that immediately enacts changes to its registry, whether additions (new registrations) or ownership/transfer changes.
An Asychronous Registry modifies the registry off-stream and updates changes periodically, perhaps twice per day.
With a Sychronous change, updates can take a few hours to be propagated across the internet and take effect, changes to an Asychronous registry will take longer to be propagated; it can take up to 72 hours for a change to propagate in extreme circumstances.