Incremental sync for an application (client application such as Confluence synchronising from Crowd), is disabled if the following condition is met:

  private void assertIncrementalSynchronisationIsAvailable(List<Directory> activeDirectories) throws IncrementalSynchronisationNotAvailableException
    {
        for (Directory directory : activeDirectories)
        {
            // No events are generated for cacheable directories that are not cached
            if (isFalse(toBooleanObject(directory.getValue(DirectoryProperties.CACHE_ENABLED))))
            {
                throw new IncrementalSynchronisationNotAvailableException("Directory '" + directory.getName() + "' is not cached and so cannot be incrementally synchronised");
            }
        }
    }

Where com.atlassian.crowd.directory.sync.cache.enabled is set to false, for the directory's attribute in cwd_directory_attribute. Considering that users and groups for an uncached directory is still cached in the DB anyway, it seems that this check does nothing but to create sync performance issues for customers with delegated directories (it will always fall back to full sync). We should remove this check altogether, or at least, set cache.enabled to always true, for delegated directories