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My initial thoughts are that the view itself is the major issue, in that it is ranking over the entire data set.

That is exactly it, the window function is applied to the whole dataset to then be filtered by CustomerId and rn=1. See my previous answer here for more information.

With the CustomerId filter predicate applied only when the data gets to the filter operator:

You could add OPTION(RECOMPILE) to the select query with the filter to see the CustomerId at runtime and filter before passing the data through the window function

SELECT * FROM vw_SyncDataRecent
WHERE CustomerId = 600528
OPTION(RECOMPILE);

Other methods as noted in my other answer are not using the view, using a tvf instead, not using a window function to do the filtering, stored procedures, ....

TVF example

CREATE FUNCTION dbo.[Fnc_SyncDataRecent]
(
    @P1 INT
)  
RETURNS TABLE
WITH SCHEMABINDING AS
RETURN
       WITH Ranked AS  (
    SELECT 
            Id, 
            CustomerId, 
            CategoryId, 
            ROW_NUMBER() OVER (
                PARTITION BY Id, CustomerId 
                ORDER BY DateCreated DESC) AS rn
        FROM dbo.SyncData AS cb
    )
    SELECT Id, CustomerId, CategoryId 
    FROM Ranked 
    WHERE rn = 1 and CustomerId = @P1
     


SELECT * FROM dbo.[Fnc_SyncDataRecent](600528);

I would also add this index:

CREATE INDEX IX_CustomerID3
ON dbo.SyncData([CustomerId])
INCLUDE(CategoryId,ID,DateCreated);

EDIT Your stored procedure

CREATE PROCEDURE [dbo].[_SyncData_Recent]
(
@CustomerId int = null
)
AS
BEGIN
    WITH Ranked AS (
        SELECT 
            Id, 
            CustomerId, 
            CategoryId, 
            ROW_NUMBER() OVER (PARTITION BY CustomerId, CategoryId ORDER BY DateCreated DESC) AS rn
        FROM SyncData AS cb
        WHERE CustomerId = @CustomerId
        AND DELETED_FLAG = 0
    )
    SELECT Id, CustomerId, CategoryId FROM Ranked WHERE rn = 1 AND CustomerId = @CustomerId
END
GO

EXEC [dbo].[_SyncData_Recent] 600528;

Would also work to apply earlier filtering, but I would add this index:

CREATE INDEX IX_CustomerID_CategoryId_DateCreated_
ON dbo.SyncData([CustomerId],CategoryId,DateCreated DESC)
INCLUDE(DELETED_FLAG,ID)
WHERE DELETED_FLAG = 0;

to remove the sort operator.

And the last filter, WHERE rn = 1 AND CustomerId = @CustomerId could just be WHERE rn = 1.