Response time increased significantly with read operations of managed users with relationship using JDBC as repo

The test preload 30K users with managed roles and managers, then either read the user w/o link expansion as managed/user/A-A-123 or w/ link expansion on roles like managed/user/A-A-123?_fields=,roles/. Between 6.5.0.3 and 7.0.0, we see mean response time(throughput drops) jumped significantly:

Attached are the idm metrics collected during the tests and comparisons between 6.5.0 and 7.0(F1 is for 6.5 and F2 is for 7.0).
Read user with link expansion on manager and read user with link expansion on reports showed similar degree response time degradation.

To reproduce the symptom:
Use the config attached and run the following tests between 6.5 and 7.0:

./run-pybot.py -v -c perf -s relationship.ReadManUsersWithRolesManager -t IDM_Read_User_With_No_Link_Expansion OpenIDM

or 

./run-pybot.py -v -c perf -s relationship.ReadManUsersWithRolesManager -t IDM_Read_User_With_Roles_Link_Expansion OpenIDM

From the IDM metrics, it looks the degradation might be from different causes, so added this ticket for JDBC repos instead using OPENIDM-14714.