The change made in
SOLR-8892 allowed for Jmx requests for MBean info to skip displaying the full contents of FieldCache entries, and just return the count.
However, it still computes all the field cache entry info but throws it away and uses only the number of entries. This can make the Jmx MBean retrieval quite slow which is not ideal for regular polling for monitoring purposes. We've typically found the Jmx call took over 1 minute to complete, and jstack output showed that building the stats for this bean was the culprit.
With this patch, the time is much reduced, usually less than 10 seconds. The response contents are unchanged.