The issue with the slow Cypher statement lies in the large number of relationships being expanded, particularly with the Neo4j Fundamentals course having a high number of enrollments. The query plan identified that starting at the (:Course) node was the best place to begin the traversal, which is sensible given the schema. However, when 40k :FOR_COURSE and 40k :HAS_ENROLMENT relationships are expanded, the query time increases significantly due to the large amount of data being checked. To improve performance, using indexes can help reduce the number of database hits. By adding a USING INDEX clause to the query, the statement was able to register only 592 total db hits in 404 ms, resulting in a 1385% performance improvement.