In your DTP logs, please check which step is taking the maximum time for each data packet. If the extraction step is taking the max time as you mention, check the following
1. What are filters being used in the DTP?
2. What semantic grouping has been defined?
Bear in mind, cube is not ideal as a source for a transformation, so loading performance will not be as optimal as it is when the source is a DSO.
Regards,
Suhas