I have an excerpt of code below which pushes data into a sql database table.

I am utilising SQL within a Visual Studio C# Console App in .NET 4.8.

The data that is being pushed is called via API, parsed from Json and populates a data table I have created within C#. The next step is to populate a table in SQL with this output.

The first part runs very quickly, in less than half a second, however the second part, below causes the integration to take several minutes to run.

I have used a similar method in other integrations I’ve built (either create/update/delete or alternatively merge) and the results have usually been quicker.

Any advice or suggestions are very welcome on what I could be doing wrong here, thank you!
C

    //sql staging connection
            SqlConnection connection = ConnectionStrings.SQLStagingArea();
            SqlCommand delEmployeeUpdate = new SqlCommand("DELETE sample.data.employeestoUpdate;", connection);
            string mergeEmployees = @"DELETE sample.data.employees
            WHERE NOT EXISTS(SELECT * FROM sample.data.employees_toUpdate WHERE employees_toUpdate.Empid = employees_toUpdate.EmpId)

            UPDATE sample.data.employees
                SET serviceNumber = employees_toUpdate.serviceNumber,
                firstName = employees_toUpdate.firstName,
                lastName = employees_toUpdate.lastName,
                homePhone = employees_toUpdate.homePhone,
                workPhone = employees_toUpdate.workPhone,
                mobilePhone = employees_toUpdate.mobilePhone,
                workMobilePhone = employees_toUpdate.workMobilePhone
        FROM sample.data.employees_toUpdate
            WHERE employees.Empid = employees_toUpdate.Empid

           INSERT INTO sample.data.employees
           SELECT DISTINCT
           empId,
           serviceNumber,
           firstName,
           lastName,
           homePhone,
           workPhone,
           mobilePhone,
           workMobilePhone
           FROM sample.data.employees_toUpdate
           WHERE
           NOT EXISTS(
           SELECT * FROM sample.data.employees
           WHERE
           employees_toUpdate.empId = sample.data.employees.empId)";

           SqlCommand mergeCmdEmployees = new SqlCommand(mergeEmployees, connection);


            using (connection)
            {
                connection.Open();
                delEmployeeUpdate.ExecuteNonQuery();
                using (var bulkCopy = new SqlBulkCopy(connection))
                {
                    foreach (DataColumn col in employeesDT.Columns)
                    {
                        bulkCopy.ColumnMappings.Add(col.ColumnName, col.ColumnName);
                    }

                    bulkCopy.BulkCopyTimeout = 600;
                    bulkCopy.DestinationTableName = "sample.data.employees_toUpdate";
                    try
                    {
                        bulkCopy.WriteToServer(employeesDT);
                    }
                    catch (Exception ex)
                    {
                        Console.WriteLine(ex.Message);
                        Console.ReadLine();
                    }
                }
                mergeCmdEmployees.ExecuteNonQuery();

                connection.Close();
            }

My expectation is that the code should run within a couple of seconds.
To optimize this I have attempted to simplify the code to see where the problem areas are – the major cause of delay appears to be the following command.
mergeCmdEmployees.ExecuteNonQuery();