Better insert performance on a table only with a clustered index or a table without any index on SqlServer 2008?

Note, I am not talking about a clustered index against a non-cluster index, I mean a clustered index comparing to no index at all for inserting performance.


I saw lots of links as below said that, a clustered index has better performance against no index even for insert operation, due to IAM/PFS/bitmap/..., but with my testing, seems no index is faster than cluster index, what's the problem?

http://stackoverflow.com/questions/7264820/removing-a-primary-key-clustered-index-to-increase-insert-performance

http://support.microsoft.com/kb/297861

my test scripts:

-------------------------------------------
--prepare table with clustered index
CREATE TABLE [dbo].[BigTable_CI](
[id] [int] IDENTITY(1,1) NOT NULL,
[BigChar] [char](4100) NOT NULL
) 
GO

CREATE CLUSTERED INDEX CIX_BigTable_CI
ON BigTable_CI(id)
GO


ALTER TABLE [dbo].[BigTable_CI] 
ADD CONSTRAINT [DF_BigTable_BigChar_CI] DEFAULT ('a') FOR [BigChar]
GO

CREATE PROCEDURE [dbo].[AddDataToBigTable_CI]
(@NumberOfRows bigint) 
AS
SET NOCOUNT ON;

DECLARE @Counter int = 0;
DECLARE @Start datetime = GETDATE();
DECLARE @End datetime;
DECLARE @ElapsedTime int = 0; 
DECLARE @RowsPerSecond int = 0;

WHILE (@Counter < @NumberOfRows)
BEGIN
INSERT INTO dbo.BigTable_CI DEFAULT VALUES;
SELECT @Counter += 1;
END; 

-- Calculate elapsed time and rows/second
SET @End = GETDATE(); 
SET @ElapsedTime = CONVERT(INTEGER, DATEDIFF (second, @Start, @End));
SET @RowsPerSecond = @NumberOfRows/@ElapsedTime;

-- Record results in local table
INSERT INTO dbo.Results
(StartTime, EndTime, ElapsedTime, NumberOfRows, RowsPerSecond) 
VALUES (@Start, @End, @ElapsedTime, @NumberOfRows, @RowsPerSecond);

RETURN;


-------------------------------------------
--prepare table without any index at all.
CREATE TABLE [dbo].[BigTable_NI](
[id] [int] IDENTITY(1,1) NOT NULL,
[BigChar] [char](4100) NOT NULL
) 
GO

ALTER TABLE [dbo].[BigTable_NI] 
ADD CONSTRAINT [DF_BigTable_BigChar_NI] DEFAULT ('a') FOR [BigChar]
GO

CREATE PROCEDURE [dbo].[AddDataToBigTable_NI]
(@NumberOfRows bigint) 
AS
SET NOCOUNT ON;

DECLARE @Counter int = 0;
DECLARE @Start datetime = GETDATE();
DECLARE @End datetime;
DECLARE @ElapsedTime int = 0; 
DECLARE @RowsPerSecond int = 0;

WHILE (@Counter < @NumberOfRows)
BEGIN
INSERT INTO dbo.BigTable_NI DEFAULT VALUES;
SELECT @Counter += 1;
END; 

-- Calculate elapsed time and rows/second
SET @End = GETDATE(); 
SET @ElapsedTime = CONVERT(INTEGER, DATEDIFF (second, @Start, @End));
SET @RowsPerSecond = @NumberOfRows/@ElapsedTime;

-- Record results in local table
INSERT INTO dbo.Results
(StartTime, EndTime, ElapsedTime, NumberOfRows, RowsPerSecond) 
VALUES (@Start, @End, @ElapsedTime, @NumberOfRows, @RowsPerSecond);

RETURN; 

-------------------------------------------
--prepare the results table
create table dbo.Results 
(
StartTime datetime, 
EndTime datetime, 
ElapsedTime int, 
NumberOfRows int, 
RowsPerSecond int
)

-------------------------------------------
--run scripts:
exec [dbo].[AddDataToBigTable_NI] 1000000
exec [dbo].[AddDataToBigTable_CI] 1000000