Major Description
Debugging performance issues within mongoMk is currently somewhat more challenging that it would have to - IMO because there is no easy logger that can be turned on to get some performance numbers. Ie currently for figuring out what's going on in mongoMk you can set the log level to TRACE for the following loggers:
org.apache.jackrabbit.oak.plugins.observation.EventGenerator org.apache.jackrabbit.oak.plugins.document.mongo.MongoDocumentStore org.apache.jackrabbit.oak.plugins.document.NodeDocument org.apache.jackrabbit.oak.plugins.document.DocumentNodeStore
while you might still not catch all interesting and crucial details.
So the suggestion is to add a dedicated logger which is used at a couple of neuralgic locations (eg in observation event generation, cache handling, diffChildren, findUncached etc)