Analytical queries require a full table scan. Some people succeed in speeding up analytical queries on JanusGraph using OLAP, check the older questions on OLAP and SparkGraphComputer and
https://docs.janusgraph.org/advanced-topics/hadoop/

A special case occurring very frequently, is counting the number of vertices for each label (you say: concept). Speeding this up is listed in the known issues:
https://github.com/JanusGraph/janusgraph/issues/926

Best wishes,   Marc