Solution : https://service.sap.com/sap/support/notes/416729 (SAP Service marketplace login required)
Key words :
sql trace shows, setting deletion flags, full table scan, 'order number' field, large number, deletion / archiving, internal order, 'client' field, table rkpf, long runtimes
Related Notes :
| 436544 | Internal order: performance with mass processing or batch |
| 334224 | Important notes for creating indexes |