I have not had much experience with DB2, but given that the relational data model and normalization was invented at IBM (Codd) and IBM's implemenation of those concepts was DB2, DB2 performing poorly with a normalized data model seems strange.

My recollection was that DB2 did not support multi version concurrency control like Oracle and Postgres did. The result was a lot of lock contention with DB2 if you were not careful. MVCC was eventually added to DB2, but by then it was too late.