I just wrote a blog post to explain the internals of edges in JanusGraph and give a few examples of how to optimize edge queries. Here is the medium blog post: JanusGraph Deep Dive (Part 3): Optimize Edge Queries
Check it out if you are interested in 1) how JanusGraph stores edge, 2) how JanusGraph handles edge query with predicate pushdowns 3) how Vertex-Centric Index works! I am happy to answer any questions here too.

Best regards,
Boxuan