Relation Between VBRP and VFSI (Points Will be awarded)

Dear guys

I want to pick the cost from table VFSI (Shipment Costs) corresponding to VBRP (Billing: Item Data).

The Only relation between them is document number fields that VBELN and VBGEL due to which VFSI is taking too much time to fetch the data.

The KNUMV field which is the key field in VFSI is not matching with value of KNUMV field present in VBRP the only field whose values are matching is VBGEL in VBRP and VBELN in VFSI.

Is there any way to retriev data fast.