Retrieve Purchase Orders

Hi,

For retrieving Purchase Orders I am using the following code

_nsService.tokenPassport = CreateTokenPassport();
TransactionSearchBasic tr = new TransactionSearchBasic()
{
type = new SearchEnumMultiSelectField()
{
@operator = SearchEnumMultiSelectFieldOperator.anyOf,
searchValue = new[] { “_purchaseOrder” },
operatorSpecified = true,
},
tranId = new SearchStringField()
{
@operator = SearchStringFieldOperator.@is,
searchValue = orderNum,
operatorSpecified = true
}
};
SearchResult searchResult = _nsService.search(tr);
return searchResult.recordList;

it works but very slow, how can I speed it up ?