Currently, the monitoring plugin caches data that is to be archived, and flushes that cache to the database once every 60 seconds, batching queries where possible. Usage of these batches is intended to reduce the overall workload of the database.
It has proven to be undesirable to have a relative long time in which archived messages are not yet available in the database. It makes have considerable impact, for instance.
Openfire should flush the cache more frequently.
Fixed in Monitoring plugin version 1.6.0.