Why does Nodbc perform very slow on native database (Nav2009)

Klaassen

After some strugles we succeeded in connecting DMS Document Management System with Nav2009R2 fdb database through NODBC, to enrich data in DMS (Content Central).
But performance is very bad. At the moment it is reading ca. 300 invoice header per hour.
We are using NO. to find recod in sale invoice header table using SELECT [fieldnames] FROM Sales Invoice Header WHERE No. = [invoice number]
Documents imported in DMS are enriched one by one through NODBC connection. NODBC logs in retrieves data logs out for each file in DMS.
Although field No. is first and only field in one of the index keys performance still is bad.
On server 2012 which runs DMS only NODBC is installed (no classic client installed, which isn't neccessary it´s not a workstation).

I used ODBC in the past to import financial data into excel sheets which worked fine even though these were flowfields.

Has someone any ideas how to increase performance. Loading history (several years) into DMS takes too much time this way.