Deletes fine

Azure

CREATE TABLE dbo.ThisWorksFine
(
    StupidSK int IDENTITY(1,1) NOT NULL PRIMARY KEY
,   BatchId int
,   Value nvarchar(50)
);

-- (422500 row(s) affected)
-- 24 seconds
INSERT INTO dbo.ThisWorksFine
(
    BatchId
,   Value
)
SELECT
    10 AS BatchId
,   LEFT(C.name,50) AS Value
FROM
    sys.columns AS C
    CROSS APPLY
        sys.columns AS C2;

-- (422500 row(s) affected)
-- 24 seconds
INSERT INTO dbo.ThisWorksFine
(
    BatchId
,   Value
)
SELECT
    30 AS BatchId
,   LEFT(C.name,50) AS Value
FROM
    sys.columns AS C
    CROSS APPLY
        sys.columns AS C2


-- (1267500 row(s) affected)
-- 1:24 
INSERT INTO dbo.ThisWorksFine
(
    BatchId
,   Value
)
SELECT
    50 AS BatchId
,   LEFT(C.name,50) AS Value
FROM
    sys.columns AS C
    CROSS APPLY
        sys.columns AS C2
    CROSS APPLY
        sys.tables AS T

-- 2112500
SELECT COUNT_BIG(1) AS rc FROM dbo.ThisWorksFine AS TWF

SELECT CAST(CURRENT_TIMESTAMP AS datetime2(7)) AS StartTime;
DELETE TWF
FROM
    dbo.ThisWorksFine AS TWF
WHERE
    TWF.BatchId = 30;
SELECT CAST(CURRENT_TIMESTAMP AS datetime2(7)) AS StopTime;