SqlCommand filter by dates are slow

Question

I have these codes(.net framework 2.0):

    string commandText =
            "SELECT TOP(@maximumRows) * " +
            "FROM " +
            "( " +
                "SELECT *, ROW_NUMBER() OVER (ORDER BY " + sortExpression.Trim() + ") as rowNumber " +
                "FROM " +
                "( " +
                    "SELECT * FROM vw_RFQPartVendor " +
                    "WHERE Rec_date >= @requestDateFrom " +
                    "AND Rec_date <= @requestDateTo ";
    if (!string.IsNullOrEmpty(rfqNumber))
    {
        commandText += "AND Regno = @rfqNumber ";
    }
    if (!string.IsNullOrEmpty(partNumber))
    {
        commandText += "AND Partno = @partNumber ";
    }
    if (!string.IsNullOrEmpty(vendorCode))
    {
        commandText += "AND vendorCode = @vendorCode ";
    }
    commandText += ")as innerDerivedTable " +
")AS outerDerivedTable " +
"WHERE outerDerivedTable.rowNumber > @startRowIndex " +
"ORDER BY " + sortExpression.Trim();

The part WHERE Rec_date >= @requestDateFrom and Rec_date <= @requestDateTo is causing it to run slower, some noticeable seconds.

How can I optimize it to make it run faster?