CA PPM 15.2 is indeed officially not supported with Oracle 12.2.0.1 database. But I would not say that you cannot upgrade as of now as I saw that you mentioned you will upgrade the PPM soon. Till then the following thing can help you, else, as per as my knowledge, you might run into different kind of performance issues.
For Oracle 12c R2 (12.2.0.1), enable the following parameter and set the optimizer to 12.1.0.2.
To improve performance, database administrators can run the following commands:
sqlplus / as sysdba
ALTER SYSTEM SET "_fix_control" = '17800514:0';
ALTER SYSTEM SET OPTIMIZER_FEATURES_ENABLE= '12.1.0.2' SCOPE=BOTH;
Exit;
Best of luck!!!