USQL Nesting TVFs and Queries is giving horrendous results

I 'think' that this problem is relating to the query optimization that Azure Data Lake Analytics does; but let's see...

I have 2 separate queries (TVFs) doing aggregations, and then a final Query to join the 2 together for final results. So ...

Table >  Header Query
Table >  Detail Query
Result = Header Query + Detail Query

To test the whole logic out, I run the minor queries separately with a filter, storing the results to file, and then use the hard files as sources for the final query; these are the total durations (minutes).

Header Query  1.4  (408 rows)
Detail Query  0.9  (3298 rows)
Final Query   0.9  (408 rows)

So I know that as a maximum, I can get my result in around 3.5 minutes. However, I don't really want to create new intermediary files. I want to use the TDFs directly to feed the final query.

With TDFs in the final query, the Job Graph gets to around 97% progress within about 1.5 minutes. But then, all hell breaks loose ! The last node is a Aggregate with 2,500 Vertices that says 16 minutes compute time. So my question ... WHY ??

Is this a case of me not understanding some fundamental concepts of how Azure works ?

So, can anyone explain what's going on? Any help appreciated.

Final Query:

@Header =
SELECT [CTNNumber],
       [CTNCycleNo],
       [SeqStart],
       [SeqEnd],
       [StartUTC],
       [EndUTC],
       [StartLoc],
       [StartType],
       [EndLoc],
       [EndType],
       [Start Step],
       [Start Ctn Status],
       [Start Fill Status],
       [EndStep],
       [End Ctn Status],
       [End Fill Status]
FROM [Play].[getCycles3]
     ("") AS X;


@Detail =
SELECT [CTNNumber],
       [SeqNo] AS [SeqNo],
       [LocationType],
       [LocationID],
       [BizstepDescription],
       [ContainerStatus],
       [FillStatus],
       [UTCTimeStampforEvent]
FROM [Play].[getRaw]
     ("") AS Z;

@result =
    SELECT
        H.[CTNNumber], H.[CTNCycleNo], H.[SeqStart], H.[SeqEnd]
        ,COUNT([D].[SeqNo]) AS [SeqCount]
        //, COUNT(DISTINCT [LocationID]) AS [#Locations]
    FROM 
        @Header AS [H]
        INNER JOIN
        @Detail AS [D]
        ON 
        [H].[CTNNumber] == [D].[CTNNumber] 
    WHERE 
        [D].[SeqNo] >= [H].[SeqStart] AND
        [D].[SeqNo] <= [H].[SeqEnd]  
    GROUP BY 
        H.[CTNNumber], H.[CTNCycleNo], H.[SeqStart], H.[SeqEnd]
    ;