Nested foreach loop slow
SearchResultCollection absaUsers = ABSAds.FindAll();
SearchResultCollection srcUsers = ds.FindAll();

foreach (SearchResult users in srcUsers)
{
    string cn = users.Properties["cn"][0].ToString();
    string sn = users.Properties["sn"][0].ToString();
    string userID = users.Properties["uid"][0].ToString();
    string description = users.Properties["PersonnelAreaDesc"][0].ToString();
    string jobCodeID = users.Properties["JobcodeID"][0].ToString();
    string CostCentreID = users.Properties["costCentreID"][0].ToString();
    string CostCentreDescription = users.Properties["CostCentreDescription"][0].ToString();
    string givenName = users.Properties["givenName"][0].ToString();
    string employmentStatus = users.Properties["EmploymentStatus"][0].ToString();
    string EmploymentStatusDescription = users.Properties["EmploymentStatusDescription"][0].ToString();

    foreach (SearchResult absaUser in absaUsers)
    {

        string absaUID = absaUser.Properties["uid"][0].ToString();
        string absaEmploymentStatus = absaUser.Properties["EmploymentStatus"][0].ToString();
        string absaEmploymentStatusDescription = absaUser.Properties["EmploymentStatusDescription"][0].ToString();
        string absaEmployeeNumber = absaUser.Properties["employeeNumber"][0].ToString();

        if (absaUID == cn && absaEmployeeNumber==userID)
        {
            Console.WriteLine("User Record Found:" + cn);
            sw.WriteLine("Modify" + "," + cn + "," + description + "," + userID + "," + givenName + "," + sn + "," + jobCodeID + "," + CostCentreID + "," + CostCentreDescription + "," + sn + "," + cn + "," + employmentStatus + "," + EmploymentStatusDescription);
            sw.Flush();
            break;
        }
    }
}

var lookup = absaUsers.Cast<SearchResult>()
                      .ToLookup(x => x.Properties["uid"][0].ToString());

foreach (SearchResult users in srcUsers)
{
    string cn = users.Properties["cn"][0].ToString();
    foreach (SearchResult matches in lookup[cn])
    {
        ...
    }
}

List<SearchResult> allResultGroups=new  List<SearchResult>();
    foreach (SearchResult absaUser in absaUsers)
    {
    resultGroups = srcUsers.Where(g => g.cn == absaUser.absaUID && absaUser.absaEmployeeNumber==g.userID ).ToList();
    }