Solution : https://service.sap.com/sap/support/notes/369658 (SAP Service marketplace login required)
Key words : 
document flowfull table scanvtfadocument flow reason, system reads document flow table vtfa, 'analyze table vtfa compute statistics', symptom long runtimes occur, addition %_hints oracle 'rule', transport number vtfa-tknum, sd document flow, transport number tknum, entire table vtfa, transport document exists
Related Notes : 
       
| 816915 | Bad performance when accessing table VTFA | 
| 564019 | Access to VTFA for the DB2 database takes long | 
| 394413 | Long runtime when displaying shipment | 
| 380848 | Bad performance when archiving SD_VTTK | 
| 129385 | Database hints in Open SQL |