Database queries runs sow during specific time

My issue is, application runs Batch everyday. When it runs during 2am to 7am it runs slow. When it runs during any other time it runs faster. We compared AWR's and observed high User I/O during prb window, but the plan is same and we ruled our any other factors like backups etc., Its container db with one pdb init. my question is why db behaving strange during that time? we also tried disabling automated maintenance tasks but no luck. Is there any chance of resource manger plan limiting user i/o during this frame? In CDB level its "Force:default_cdb_plan" and