SQLクエリのパフォーマンス要因（異なる列の指定による変化）

SQLに似たような性質（データ型:decimal）の列に条件を置き換えてSQLクエリを回したところ、大幅にパフォーマンスが異なり、困惑しております。

前提・実現したいこと

クエリのパフォーマンスの悪化原因を明らかにし、パフォーマンスの大幅な改善を図りたい。

発生している問題・エラーメッセージ

SQLに似たような性質（データ型:decimal）の列に条件を置き換えてSQLクエリを回したところ、大幅にパフォーマンスが異なり、困惑しております。

DTP_POINTS
Decimal(16,０) null) - NOT ALLLOW NULL
処理時間５分

DTP_POINTS_BASIC
Decimal(18,０）not null) - ALLLOW NULL
処理時間２時間以上

該当のソースコード

２つあります

違いは指定した列の違いになります。指定列が[DTP_POINTS]と [DTP_POINTS_BASIC] だけが以下のコードで異なります。

DTP_POINTS
Decimal(16,０) null) - NOT ALLLOW NULL
処理時間５分

DTP_POINTS_BASIC
Decimal(18,０）not null) - ALLLOW NULL
処理時間２時間以上

**__処理時間５分__**

WITH BASE AS (SELECT [DTP_MEMBER_ID] 
FROM  
[DWH_PP].[dbo].[DAILY_TRX_PLMS_2018] 
WHERE  
[DTP_COMPANY] = '001' AND 
MONTH([DTP_TRX_DATE]) = '5' AND  
YEAR([DTP_TRX_DATE]) = '2018' AND 
[DTP_TRANSACTION_CODE] = 'POI') 

SELECT 
(SELECT COUNT(DISTINCT [DTP_MEMBER_ID]) FROM BASE) AS 'UNIQUE_MEMBER_PREVIOUS_MONTH', 
COUNT(DISTINCT BASE2.[DTP_MEMBER_ID]) AS 'UNIQUE_MEMBER_NEXT_MONTH', 
SUM(BASE2.[DTP_AMOUNT]) AS 'AMOUNT_AT_FIRST_TRX', 
SUM(BASE2.[DTP_POINTS]) AS 'POINT_ISSUE_AT_FIRST_TRX', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 500 ELSE BASE2.[DTP_POINTS] END) AS 'BONUS_POINT_2X', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 750 ELSE FLOOR(BASE2.[DTP_POINTS] * 1.5) END)  AS 'BONUS_POINT_3X', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 1000 ELSE FLOOR(BASE2.[DTP_POINTS] * 2) END)  AS 'BONUS_POINT_4X', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 1250 ELSE FLOOR(BASE2.[DTP_POINTS] * 2.5) END)  AS 'BONUS_POINT_5X' 

FROM ( 
 SELECT
 [DTP_MEMBER_ID],
 [DTP_AMOUNT],
 [DTP_POINTS],
 ROW_NUMBER() OVER (PARTITION BY [DTP_MEMBER_ID] ORDER BY [DTP_TRX_DATE] ASC) AS 'TRX_COUNT'
 FROM [DWH_PP].[dbo].[DAILY_TRX_PLMS_2018]
 WHERE [DTP_COMPANY] = '001' AND
 MONTH([DTP_TRX_DATE]) = '6' AND 
 YEAR([DTP_TRX_DATE]) = '2018' AND
 [DTP_TRANSACTION_CODE] = 'POI' AND [DTP_MEMBER_ID] IN 
 (SELECT [DTP_MEMBER_ID] FROM BASE)) AS BASE2
WHERE TRX_COUNT = 2 


**__処理時間２時間以上__**

WITH BASE AS (SELECT [DTP_MEMBER_ID] 
FROM  
[DWH_PP].[dbo].[DAILY_TRX_PLMS_2018] 
WHERE  
[DTP_COMPANY] = '001' AND 
MONTH([DTP_TRX_DATE]) = '5' AND  
YEAR([DTP_TRX_DATE]) = '2018' AND 
[DTP_TRANSACTION_CODE] = 'POI') 

SELECT 
(SELECT COUNT(DISTINCT [DTP_MEMBER_ID]) FROM BASE) AS 'UNIQUE_MEMBER_PREVIOUS_MONTH', 
COUNT(DISTINCT BASE2.[DTP_MEMBER_ID]) AS 'UNIQUE_MEMBER_NEXT_MONTH', 
SUM(BASE2.[DTP_AMOUNT]) AS 'AMOUNT_AT_FIRST_TRX', 
SUM(BASE2.[DTP_POINTS_BASIC]) AS 'POINT_ISSUE_AT_FIRST_TRX', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 500 ELSE BASE2.[DTP_POINTS_BASIC] END) AS 'BONUS_POINT_2X', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 750 ELSE FLOOR(BASE2.[DTP_POINTS_BASIC] * 1.5) END)  AS 'BONUS_POINT_3X', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 1000 ELSE FLOOR(BASE2.[DTP_POINTS_BASIC] * 2) END)  AS 'BONUS_POINT_4X', 
SUM(CASE WHEN BASE2.[DTP_AMOUNT] >= 100000 THEN 1250 ELSE FLOOR(BASE2.[DTP_POINTS_BASIC] * 2.5) END)  AS 'BONUS_POINT_5X' 

FROM ( 
 SELECT
 [DTP_MEMBER_ID],
 [DTP_AMOUNT],
 [DTP_POINTS_BASIC],
 ROW_NUMBER() OVER (PARTITION BY [DTP_MEMBER_ID] ORDER BY [DTP_TRX_DATE] ASC) AS 'TRX_COUNT'
 FROM [DWH_PP].[dbo].[DAILY_TRX_PLMS_2018]
 WHERE [DTP_COMPANY] = '001' AND
 MONTH([DTP_TRX_DATE]) = '6' AND 
 YEAR([DTP_TRX_DATE]) = '2018' AND
 [DTP_TRANSACTION_CODE] = 'POI' AND [DTP_MEMBER_ID] IN 
 (SELECT [DTP_MEMBER_ID] FROM BASE)) AS BASE2
WHERE TRX_COUNT = 2 

試したこと

WHERE条件などで IS NOT NULLなどの条件を記載しましたが、特に変わらず（といいうよりDB内にNULLは存在せず）。

補足情報（FW/ツールのバージョンなど）

MS SQL Server