Get adjacent/connected RIDs as Strings

[BK]

Given a "source" vertex/RID, what's the fastest way to get the RIDs of adjacent vertices (by edge label) as String values?  I don't need the full record for each adjacent vertex.  I'm using the Java API, and the approaches I've tried - Blueprints API and direct queries - are slow.

For background: the overall goal is convert source vertices and the String values of their adjacent RIDs into a serialized representation for a web API client.  The transfer objects I'm creating have fields corresponding to each field in the source vertices, and List<String> fields for each edge label to hold the RID strings for vertices connected by edges with those labels.  The graph has over 400k vertices and over 1M edges.

Thanks!