Hello -
Check for any routines in your DTP filters. The DTP has to generate the filter criteria before performing the extraction. If there is complex logic in a routine, you will see the additional time in this step.
It could also be that you need to clean-up your basis tables - reference note 1829728 for BW housekeeping task list. Also consider archiving old requests with RSREQARCH.
Optimization of Data Retention - Data Warehouse Management - SAP Library
Regards,
Tony