Dan Geringer, Senior Software Development Manager at Oracle, was on site this week working with our DBA, Tim Gerber, and they steered me towards documentation about the spatial vector acceleration available in 12c. The parameter is documented in the Oracle Spatial and Graph Developers Guide:
To accelerate the performance of spatial operators, it is recommended that you enable the Vector Performance Accelerator (VPA) feature by setting the SPATIAL_VECTOR_ACCELERATION database system parameter to the value
Simply run the following command from a suitably privileged account.
SQL> ALTER SYSTEM SET SPATIAL_VECTOR_ACCELERATION=TRUE SCOPE=BOTH; System altered.