Hi,
Query variables are not passed only to the virtual provider by default, i.e enter dataset of the extractor is pulled the data manager and then it is filtered out which could the issue for the long runtimes.
If the datasource of the virtual provider has selections then we could passed those from the query to the datasource while query execution at the inverse start routine and reduce the amount of data actual pulled by the virtual provider and hence improve the performance of the query.
Below is the link to the sap documentation on inverse routine.
http://help.sap.com/saphelp_erp60_sp/helpdata/en/45/f5af4a4db53482e10000000a1553f6/content.htm
Thanks,
Arun