ABAP Function Module alternative to MELB transaction

Hello,

I figuring out how to get Purchase orders for a specific Tracking number with ABAP function module with SapNCo (.Net Connector).

I currently do that with FM BAPI_PO_GETITEMS. I fill in TRACKINGNO, DOC_DATE and invoke my destination. But because it takes unreasonable amount of time to execute, I think there should be another way.

If I try to do the same with transaction MELB or ME2B via SAP GUI, the results appear very fast.

Do anyone know if there is any better BAPI "on the market" to do my job?

Thank you