Solution : https://service.sap.com/sap/support/notes/138794 (SAP Service marketplace login required)
Key words : 
runtime additional key words data records, entry 'data extraction - maintain control parameters, symptom slow extraction, make similar settings, select 'source systems', data records, entry 'number, data loading, data transfer', data packets
Related Notes : 
       
| 365762 | Loading data: Pre-analysis of (performance)problems | 
| 157652 | Main memory requirement for extraction from R/3 | 
| 156319 | Collective note performance BW 1.2B |