EM13: Performance issue querying SYSMAN.GC_METRIC_STR_VALUES

I have a Metric Extension in EM 13.2.0.0.0 whose values are strings. But querying the SYSMAN.GC_METRIC_STR_VALUES view on the 12.2.0.1.0 OMS is very slow, taking 2 minutes to return the first 500 rows from the simple cursor:

SELECT * FROM SYSMAN.GC_METRIC_STR_VALUES When trying to return metric values in past versions of EM, I've filtered the queries with various columns like:

SELECT
    *
FROM
    sysman.gc_metric_str_values
WHERE
    entity_type = 'oracle_database'

    AND entity_name = 'myoradb'
    AND entity_guid = '8C686DE17F33936EAFB49CA9AE1A0E6D'
    AND metric_group_name = 'ME$myext'
    AND metric_column_name = ('fun_text')
    AND metric_column_guid = '6C80A9B8B5A3AD57D9412874F5B0BB2D'