Solution : https://service.sap.com/sap/support/notes/496167 (SAP Service marketplace login required)
Key words : 
long runtimes occur, time slot search, attached correction instruction, correction instructions, database db2, transaction nr16, terms nr16, visit scheduling, n_vnapp_npap reason, problem exists
Related Notes : 
       
| 162034 | DB2/390: Database hints in Open SQL | 
| 113008 | DB2/390: Maintaining Catalog Statistics |