Audit - AuditTrailService is slow when there are many values ​​in the AuditDataItemPersistent table

Hello to all,
my application, use web project, XAF Framework 20.1.7.

I have unexpected behavior of the AUDIT module, especially the time it took to insert the record.

Currently I have this amount of records in the tables related to the AUDIT (AuditTrail)(abaut 5 milion record):

SQL
SELECT count(*)   FROM [EAMS_OL_SL].[dbo].[AuditDataItemPersistent]  -- 5.053.084
SELECT count(*)   FROM [EAMS_OL_SL]. [dbo].[AuditedObjectWeakReference] -- 465.889
SELECT count(*)   FROM [EAMS_OL_SL].  [dbo].[XPWeakReference]-- 2.954.143

I did this test (for one record):
A) In the case of the aforementioned record quantity: the time to insert this record is 3-4 seconds (for one record);

C#
using (UnitOfWork UW = Helpers.XpoHelperAudit.GetNewUnitOfWork())
           {
               //AuditTrailService.Instance.BeginSessionAudit(UW, AuditTrailStrategy.OnObjectChanged);
               //*******************************************************************************************  
             ..........
             ........
               if (vIEPlantList != null)
               {
                   XPQuery<IEValueList> qIEValueList = new XPQuery<IEValueList>(UW);
                   int cont = qIEValueList.Where(w => w.IEPlantList.Oid == vIEPlantList.Oid && w.DataValueString == Datacsv).Count();
                   //&& w.DataValue > dataValore.AddMinutes(-1) && w.DataValue < dataValore.AddMinutes(1)  ).Count();
                   if (cont == 0)  /// se il valore non e' stato già inserito 
                   {
                       //Console.WriteLine("file csv: Value of i:{0} -  {1}  {2}", riga, Datacsv, Valorecsv);
                       Console.WriteLine("  file csv:{0} Riga:{1}, Data:{2}, Valore:{3}", PathFileName, riga, Datacsv, Valorecsv);
                       /// carico la lista dei valori
                       IEValueList myNewIEValueList = new IEValueList(UW);
                       //myNewIEValueList.DataValue = dataValore;
                       myNewIEValueList.DataValueString = Datacsv;
                       myNewIEValueList.Value = Valorecsv;
                       myNewIEValueList.IEPlantList = vIEPlantList;
                       myNewIEValueList.DataAggiornamento = DateTime.Now;
                       myNewIEValueList.Descrizione = string.Format("File Import: {0}", worksheet.Name);
                       myNewIEValueList.Save();
                       UW.CommitChanges();
                   }
                   else
                   {
                       Console.WriteLine("valore già inserito, file csv:{0} Riga:{1}, Data:{2}, Valore:{3}", PathFileName, riga, Datacsv, Valorecsv);
                       myLogger.writeTestLog(string.Format("valore già inserito, file {0} riga {1}, Applicazione {2}", PathFileName, riga, pApplicazione));
                   }
               }                    
               //*******************************************************************************************
               Console.WriteLine("  file  DateTime.Now):{0} ", DateTime.Now);
               //AuditTrailService.Instance.QueryCurrentUserName += OnAuditTrailServiceInstanceQueryCurrentUserName;
               //AuditTrailService.Instance.SaveAuditData(UW);
               //AuditTrailService.Instance.QueryCurrentUserName -= OnAuditTrailServiceInstanceQueryCurrentUserName;
               Console.WriteLine("  file  DateTime.Now):{0}", DateTime.Now);
           }

B) In the case of the REDUCED record quantity, I have deleted 90% of the records and the time to insert this record is less than one second (same code as above);

it does not seem to me (from my knowledge) that the insertion time of the records in the table, from the point of view of the t-sql code, varies in this way according to the records count in the table.

so I assume that the "AuditTrail" module executes queries before / after inserting records on the "AuditDataItemPersistent" table.

if so, perhaps an index to be created is missing or the insertion procedure could be improved.

I await your clarification.