Query Slow down due to structure of WHERE clause - sql

Question

I have a query that creates an #TABLE of a population of interest. It's structure is like this:
DECLARE #SepsisTbl TABLE (
PK INT IDENTITY(1, 1) PRIMARY KEY
, Name VARCHAR(500)
, MRN INT
, Account INT
, Age INT -- Age at arrival
, Arrival DATETIME
, Triage_StartDT DATETIME
, Left_ED_DT DATETIME
, Disposition VARCHAR(500)
, Mortality CHAR(1)
);
WITH Patients AS (
SELECT UPPER(Patient) AS [Name]
, MR#
, Account
, DATEDIFF(YEAR, AgeDob, Arrival) AS [Age_at_Arrival]
, Arrival
, Triage_Start
, TimeLeftED
, Disposition
, CASE
WHEN Disposition IN (
'Medical Examiner', 'Morgue'
)
THEN 'Y'
ELSE 'N'
END AS [Mortality]
FROM SMSDSS.c_Wellsoft_Rpt_tbl
WHERE Triage_Start IS NOT NULL
AND (
Diagnosis LIKE '%SEPSIS%'
OR
Diagnosis LIKE '%SEPTIC%'
)
)
INSERT INTO #SepsisTbl
SELECT * FROM Patients
From this point forward I have 5 more queries of the same sort that are looking for different types of orders that I then LEFT OUTER JOIN onto this table. My question is, why does my performance degrade so much when I change the where clause of the tables from this:
AND A.Account IN (
SELECT Account
FROM SMSDSS.c_Wellsoft_Rpt_tbl
WHERE (
Diagnosis LIKE '%SEPSIS%'
OR
Diagnosis LIKE '%SEPTIC%'
)
to this:
AND A.Account IN (
SELECT Account
FROM #SepsisTbl
)
The run time goes from 2.5 minutes to over 10 minutes with still no results. The CTE itself runs as fast as I can press F5.
Thank you,

Answer 1

I suspect that the problem is because the table variable doesn't have an index on Account. If you add an index on Account then I would expect better performance.
See the answer to this question for details on how to add an index: Creating an index on a table variable