Solution : https://service.sap.com/sap/support/notes/420681 (SAP Service marketplace login required)
Key words : 
additional key words atpfield, repeating table entries, performance problems occur, data consistency errors, larger sales orders, sales order interface, -apo-relevant materials, sales order, apo-relevant, system fills
Related Notes : 
       
| 609435 | Composite SAP note: Performance in backorder processing | 
| 417598 | Posting of sales orders with several items | 
| 375193 | Backorder processing: Composite SAP note on posting | 
| 370601 | Composite SAP note: APO 3.0 and 3.1 performance |