In
src/services/org/opennms/netmgt/config/NotificationManager.java
a select is frequently made on notifications:
explain SELECT eventid FROM notifications WHERE eventuei=?
The eventuei field lacks an index, so there's always a sequential scan:
opennms=# explain SELECT eventid FROM notifications WHERE eventuei=15;
QUERY PLAN
---------------------------------------------------------------- Seq Scan on notifications (cost=0.00..413.68 rows=22 width=4)
Filter: ((eventuei)::text = '15'::text)

With index:
opennms=# explain SELECT eventid FROM notifications WHERE eventuei=15;
QUERY PLAN
-------------------------------------------------------------------------------------------------- Index Scan using notifications_eventuei_idx on notifications (cost=0.00..77.31
rows=20 width=4)
Index Cond: ((eventuei)::text = '15'::text)

So may I suggest to
create index notifications_eventuei_idx on notifications (eventuei);