Hi. I have two questions just to get into the right direction:
Is the request processed this slow all the time or is it on the first interaction with the application? I ask this because when making the first query to the database via SDN the driver will get also create its very first connection and this takes time. Of course usually not 6 seconds but it could add up to the response time, you are observing.
Second: Because I cannot see the payload in your question completely, could you give me a rough number of related entities, you want to persist? I assume right now that this POST request does add something to an already existing node that already has a lot of relationships, but want to be sure.
Something you could do to improve the time it takes SDN to update the data is to introduce RelationshipProperties in your domain definition. The update for domains with a lot of relationships usually takes a lot of time because SDN will remove and re-create (if needed) all relationships. This is due to the intentionally non-existing cache to track changes between loading and persistence. Using RelationshipProperties will make relationships no longer anonymous but require those classes to define an id field. This field is used to determine if a relationship is new and needs to get created, or if it got removed from the Java model and also needs to get removed from the database. In the end there are far much less delete and create operations on relationships than before.