XmlDocument vs StringBuilder

Потестил перфоманс создания xml через XmlDocument и StringBuilder.
В моем приложении результаты получились такие:

XmlDocument: 0.2151
StringBuilder: 0.3114

В моем случае если записывать в Xml нечего, то надо было вернуть string.Empty. Для этого случая результаты такие:

XmlDocument: 0.00778
StringBuilder: 0.01386

кусочек кода на всякий случай:

        /// <summary>
        /// Creates xml with calculated volume share values.
        /// </summary>
        /// <param name="suppliersData">Collection with calculated values.</param>
        /// <param name="startIndex">Position of first supplier to be added into xml.</param>
        /// <param name="endIndex">Position of last supplier to be added into xml.</param>
        /// <returns>Xml with calculated data || empty string if there is no values to be loaded into db for specifeid portion of suppliers.</returns>
        private static string GetVolumeShare(SupplierDataCollection suppliersData, int startIndex, int endIndex)
        {
            XmlDocument xmlDoc = new XmlDocument();
            XmlNode root = xmlDoc.CreateElement("root");
            for (int i = startIndex; i <= endIndex; i++)
            {
                SupplierData supplierData = suppliersData[i];
                foreach (SupplierYearData supplierYearData in supplierData.YearVolumes)
                {
                    foreach (SupplierCSData supplierCSData in supplierYearData.CSVolumes)
                    {
                        if (supplierCSData.CSVolumeShare > 0)
                        {
                            XmlNode child = xmlDoc.CreateElement("vs");
                            XmlAttribute childAttr = xmlDoc.CreateAttribute("SRMSId");
                            childAttr.Value = supplierData.SupplierId.ToString();
                            child.Attributes.Append(childAttr);
                            childAttr = xmlDoc.CreateAttribute("CS");
                            childAttr.Value = supplierCSData.CSId.ToString();
                            child.Attributes.Append(childAttr);
                            childAttr = xmlDoc.CreateAttribute("year");
                            childAttr.Value = supplierYearData.Year.ToString();
                            child.Attributes.Append(childAttr);
                            childAttr = xmlDoc.CreateAttribute("value");
                            childAttr.Value = supplierCSData.CSVolumeShare.ToString();
                            child.Attributes.Append(childAttr);
                            root.AppendChild(child);
                        }
                    }
                }
            }
            if (root.ChildNodes.Count > 0)
                xmlDoc.AppendChild(root);
            string xmlData = xmlDoc.InnerXml;
            return xmlData;
        }

        private static string GetVolumeShareXml(SupplierDataCollection suppliersData, int startIndex, int endIndex)
        {
            StringBuilder sbChilds = null;
            for (int i = startIndex; i <= endIndex; i++)
            {
                SupplierData supplierData = suppliersData[i];
                foreach (SupplierYearData supplierYearData in supplierData.YearVolumes)
                {
                    foreach (SupplierCSData supplierCSData in supplierYearData.CSVolumes)
                    {
                        if (supplierCSData.CSVolumeShare > 0)
                        {
                            if (sbChilds == null)
                                sbChilds = new StringBuilder();
/*
                            sbChilds.Append(
                                string.Format("<vs SRMSId=\"{0}\" CS=\"{1}\" year=\"{2}\" value=\"{3}\" />", 
                                supplierData.SupplierId, 
                                supplierCSData.CSId, 
                                supplierYearData.Year, 
                                supplierCSData.CSVolumeShare
                                )
                            );
*/
                            sbChilds.Append("<vs SRMSId=\"");
                            sbChilds.Append(supplierData.SupplierId);
                            sbChilds.Append("\" CS=\"");
                            sbChilds.Append(supplierCSData.CSId);
                            sbChilds.Append("\" year=\"");
                            sbChilds.Append(supplierYearData.Year);
                            sbChilds.Append("\" value=\"");
                            sbChilds.Append(supplierCSData.CSVolumeShare);
                            sbChilds.Append("\" />");
                        }
                    }
                }
            }
            StringBuilder sbRoot = null;
            if (sbChilds != null)
            {
                sbRoot = new StringBuilder();
                sbRoot.Append("<root>");
                sbRoot.Append(sbChilds);
                sbRoot.Append("</root>");
            }
            return sbRoot != null ? sbRoot.ToString() : string.Empty;
        }

П.С.: замена StringBuilder на string.Format никакого эффекта не дало, как собсно я и ожидал.