Error in loading data using DB2 load command in DB2 10.5
Have you run runstats recently for at least the relevant tables and indexes?
Are you sure you have index(es) to support your monthly batch?
Have you checked the bufferpool hit ratio when the batch is running?
db2top and db2advis may help you answering these questions.

The secret optimizer was surely changed and it may see tiny details differently than the previous version, that can cause dramatic differences. It's always a good idea to pamper it with runstats when unexplainable slowdown is seen and check the rest when there is no benefit.