How fast is immediately?  A well dimensioned cassandra or scylladb cluster (with its own block cache!) should be able to serve requests at the ms level.

https://www.scylladb.com/2017/09/18/scylla-2-0-workload-conditioning/

You only run into trouble with queries that ask for tens or hundreds of vertices, but you can ask if it is reasonable to be realtime at the ms level for such large queries.

Best wishes,     Marc