As of date queries - sql-server

Question

I'm trying to implement similar functionality to temporal tables
https://msdn.microsoft.com/en-us/library/dn935015.aspx
Due to some complex business requirements, I cannot use SQL Server's temporal tables, so I'm trying to implement my own version.
Basically, I need a way to be able to determine what is the data in a specific point in time.
The table will store deltas, and a bitmask to determine what columns actually changed (changing the data to NULL is a valid scenario, that's why we need this update mask)
I do have a prototype working, but my query seems very expensive. Instead of one subquery per column, I've been trying to combine them into just 1 query (with a cursor-like behavior).
Tables:
CREATE TABLE user_efdt
(
user_id INT not null,
date_effective DATE not null,
first_name NVARCHAR(100),
last_name NVARCHAR(100),
address1 NVARCHAR(100),
position NVARCHAR(100),
modified_fields VARBINARY(128) NOT NULL
)
ALTER TABLE user_efdt
ADD CONSTRAINT PK_user_efdt
PRIMARY KEY CLUSTERED (user_id, date_effective);
GO
Data setup:
DECLARE #first_name_bit INT
DECLARE #last_name_bit INT
DECLARE #address1_bit INT
DECLARE #position_bit INT
SET #first_name_bit = 3
SET #last_name_bit = 4
SET #address1_bit = 5
SET #position_bit = 6
-- user john does gets added to system on 1-may-16 with first name and last name
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (1000, '1-may-16', 'john', 'doe', null, null, CAST(POWER(2, #first_name_bit) + POWER(2, #last_name_bit) AS VARBINARY))
-- user mary ann gets added to system on 1-may-16 with first name, last name, address and position
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (2000, '1-may-16', 'mary', 'ann', '123 main st', 'manager', CAST(POWER(2, #first_name_bit) + POWER(2, #last_name_bit) + POWER(2, #address1_bit) + POWER(2, #position_bit) AS VARBINARY))
-- john doe gets address updated on 1-apr-16
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (1000, '1-apr-16', null, null, '456 el dorado st', null, CAST(POWER(2, #address1_bit) AS VARBINARY))
-- john doe gets position updated
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (1000, '1-mar-16', null, null, null, 'engineer', CAST(POWER(2, #position_bit) AS VARBINARY))
-- john doe gets position updated again
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (1000, '1-jun-16', null, null, null, 'engineer 2', CAST(POWER(2, #position_bit) AS VARBINARY))
-- john doe gets position updated to NULL (erased)
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (1000, '1-jul-16', null, null, null, null, CAST(POWER(2, #position_bit) AS VARBINARY))
-- mary ann gets address updated
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (2000, '1-jun-16', null, null, '1443 hoover st', null, CAST(POWER(2, #address1_bit) AS VARBINARY))
-- mary ann gets position updated
INSERT INTO user_efdt (user_id, date_effective, first_name, last_name, address1, position, modified_fields)
VALUES (2000, '1-jul-16', null, null, null, 'manager 2', CAST(POWER(2, #position_bit) AS VARBINARY))
Here's my version of the query:
CREATE PROCEDURE as_of_date_query (#date DATE)
AS
BEGIN
DECLARE #first_name_bit INT
DECLARE #last_name_bit INT
DECLARE #address1_bit INT
DECLARE #position_bit INT
SET #first_name_bit = 3
SET #last_name_bit = 4
SET #address1_bit = 5
SET #position_bit = 6
SELECT
q1.date_effective, fn.val first_name, ln.val last_name,
addr.val address1, pos.val position
FROM
(SELECT
user_id, MAX(date_effective) date_effective
FROM
user_efdt
WHERE
date_effective <= #date
GROUP BY
user_id) q1
LEFT JOIN
(SELECT DISTINCT
user_id,
FIRST_VALUE(first_name) OVER (PARTITION BY user_id ORDER BY date_effective DESC) val
FROM
user_efdt
WHERE
(modified_fields & POWER(2, #first_name_bit)) > 0
AND date_effective <= #date) fn ON q1.user_id = fn.user_id
LEFT JOIN
(SELECT DISTINCT
user_id,
FIRST_VALUE(last_name) OVER (PARTITION BY user_id ORDER BY date_effective DESC) val
FROM
user_efdt
WHERE
(modified_fields & POWER(2, #last_name_bit)) > 0
AND date_effective <= #date) ln ON q1.user_id = ln.user_id
LEFT JOIN (
SELECT DISTINCT user_id, FIRST_VALUE(address1) OVER (PARTITION BY user_id ORDER BY date_effective DESC) val
FROM user_efdt
WHERE (modified_fields & POWER(2, #address1_bit)) > 0
AND date_effective <= #date
) addr ON q1.user_id = addr.user_id
LEFT JOIN (
SELECT DISTINCT user_id, FIRST_VALUE(position) OVER (PARTITION BY user_id ORDER BY date_effective DESC) val
FROM user_efdt
WHERE (modified_fields & POWER(2, #position_bit)) > 0
AND date_effective <= #date
) pos ON q1.user_id = pos.user_id
END
expected results:
EXEC as_of_date_query '1-mar-16'
-- 2016-03-01 NULL NULL NULL engineer
EXEC as_of_date_query '5-apr-16'
-- 2016-04-01 NULL NULL 456 el dorado st engineer
EXEC as_of_date_query '15-may-16'
-- 2016-05-01 john doe 456 el dorado st engineer
-- 2016-05-01 mary ann 123 main st manager
EXEC as_of_date_query '5-jun-16'
-- 2016-06-01 john doe 456 el dorado st engineer 2
-- 2016-06-01 mary ann 1443 hoover st manager
EXEC as_of_date_query '1-jul-16'
-- 2016-07-01 john doe 456 el dorado st NULL
-- 2016-07-01 mary ann 1443 hoover st manager 2