With any performance issue, the first step is to look at the code and understand the process. Given table structure/size/indexes, an experienced candidate should be able to suggest whether the process in the code is appropriate, and where any performance bottlenecks might be expected.
Look at the V$ tables to identify any poor performing SQL (either a single SQL that takes a long time, or one that happens too often).
If there's no problem in the SQL, then you can identify the location of PL/SQL problems using DBMS_PROFILER.