why does using the same c# code to change password in AD in different computer make different speed?

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C#

ASP.NET

.NET

I used the class in System.DirectoryServices to change password in AD. The code like this:

DirectoryEntry _directoryEntry = new DirectoryEntry(ldapPath, user, pwd, AuthenticationTypes.Secure);
    public bool ChangePassword(string userPath, string newPassword)
    {
        try
        {
            if (userPath != null && _directoryEntry != null)
            {
                _directoryEntry.Path = userPath;
                //Set the password
                _directoryEntry.Invoke("SetPassword", new object[] { newPassword });
                _directoryEntry.CommitChanges();
                return true;
            }
        }
        catch (Exception ex)
        {
            //Invalid Login or the domain controller is not contactable
            throw ex;
        }
        finally
        {
            _directoryEntry.Close();
            _directoryEntry = null;
        }
        return false;
    }

I executed these codes on different computer. The time spent from several ms to several seconds.

why does the same code executed in different Environment to change password in AD spent different time? I have spent a lot of time in dealing this problem but still no result. Can anybody tell me? Thank you very much!!!!!