Hi Jalina
The step that's taking more time in this case is transformation only. So Number Range buffering will not be any help.
Have you done any changes to the transformation recently?If not, could you please explain since when this particular dtp is taking more time. (Just to know if it has ever ran in less time)
As you have already mentioned, there is no routines in transformation, but could you please share the screenshot of the transformation rules as well.
Do you have any filters in the DTP? Share the header tab screenshot of the DTP as well please.
Regards
Karthik