Oracle 12c query performance varies based on schema/user
You can try to check what the current consumer group for the different sessions are. I.e. log in as SAMPLE and USER1 and run

SELECT SID,SERIAL#,USERNAME,RESOURCE_CONSUMER_GROUP FROM V$SESSION
    WHERE USERNAME IN ('SAMPLE','USER1','DBA1');


You can also check what the actual plans are assigned as DBA:

SELECT * FROM DBA_RSRC_CONSUMER_GROUP_PRIVS
    WHERE GRANTEE IN ('SAMPLE','USER1','DBA1');


This is designed to grant different resources for users (and admins) and might explain what you see. The plan should actually be shared across all executing users (it remembers the user which parsed it first). Just to be sure, auto trace the execution for the two different users.