Excessive "latch: row cache objects" waits with simple INSERT statement

we have a strange performance problem in one of our test environments running Oracle 12.1 (to be upgraded soon, but not soon enough to ignore the problem).

A simple INSERT into a global temp table causes excessive "latch: row cache objects" wait events (contributing > 80% of the DB time in the AWR report).

    insert into lookup_table (lookup_value) values (:1 );

The table definition is pretty simple:

  CREATE GLOBAL TEMPORARY TABLE LOOKUP_TABLE

   (	"LOOKUP_VALUE" VARCHAR2(255 CHAR), 
	 PRIMARY KEY ("LOOKUP_VALUE") ENABLE

+----------------------+--------------+----------+-----------+----------+----------+-------------+


| Cache                | Get Requests | Pct Miss | Scan Reqs | Pct Miss | Mod Reqs | Final Usage |


+----------------------+--------------+----------+-----------+----------+----------+-------------+


| dc_awr_control       | 78           | 0.00     | 0         |          | 2        | 1           |

| dc_files             | 64           | 0.00     | 0         |          | 0        | 4           |

| dc_global_oids       | 9,085        | 0.00     | 0         |          | 0        | 439         |

| dc_histogram_data    | 492,981      | 0.05     | 0         |          | 0        | 6,631       |

| dc_histogram_defs    | 740,310      | 0.05     | 0         |          | 353      | 10,124      |

| dc_objects           | 541,488      | 0.06     | 0         |          | 240      | 4,697       |

| dc_profiles          | 769          | 0.00     | 0         |          | 0        | 2           |

| dc_props             | 3,829        | 0.00     | 0         |          | 0        | 6           |

| dc_rollback_segments | 16,087       | 0.00     | 0         |          | 0        | 303         |

| dc_segments          | 248,107      | 0.03     | 0         |          | 28       | 4,664       |

| dc_sequences         | 2,249        | 0.09     | 0         |          | 2,249    | 15          |

| dc_tablespaces       | 548,284      | 0.00     | 0         |          | 0        | 13          |

| dc_users             | 817,470      | 0.00     | 1,780     | 0.00     | 0        | 415         |

| outstanding_alerts   | 8            | 0.00     | 0         |          | 0        | 3           |


| sch_lj_oids          | 164          | 3.66     | 0         |          | 0        | 36          |


+----------------------+--------------+----------+-----------+----------+----------+-------------+

I have found references in MOS that "latch: row cache objects" might be related to the DB user having system privileges. The user did have the SELECT_CATALOG_ROLE role granted, so I revoked that but to no avail.

MOS also contains documents that state that this could happen with VPD enabled, which we don't have. But I wonder if a global temp table uses that somehow to isolate the content of the table between sessions.

The server has 32 CPUs and the AWR report shows a load of 5 seconds DB Time per second (so it's not CPU bound), 37 transactions/s and 360 executes/s

I don't have any ideas on how to investigate this further.

Any ideas on what I could investigate further to find the root cause?

Thanks
Thomas

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