Hi Vinayak,
Speeding up your query depends on your setup. 15.000 vertices/second is already fast. Is this the janusgraph inmemory backend? Or ScyllaDB?
In a perfect world, not there yet, your query would profit from parallelization (OLAP). JanusGraph supports both the withComputer() and withComputer(SparkGraphComputer) start steps, but the former is undocumented and the performance gains of the latter are often disappointing.
Best wishes, Marc