Here is the code I used:

CREATE FUNCTION dbo.TallyN (

@EndNumber bigint

)

RETURNS TABLE

AS RETURN

(-- Inline Tally table producing a number sequence from 1 to @SAMPLE_SIZE

WITH T(N) AS (SELECT N FROM (VALUES (0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) AS X(N))

SELECT TOP(@EndNumber) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) AS N

FROM T T1,T T2,T T3,T T4,T T5,T T6,T T7,T T8,T T9

)

GO

SELECT n.N ID, STUFF(

(SELECT ',' + CONVERT(VARCHAR(20), n3.RandomN)

FROM

--picking a random quantity of random numbers per ID

(SELECT TOP 1 N topn FROM dbo.TallyN(30) WHERE n.N > 0 ORDER BY NEWID() ) n2

-- now picking the random numbers themselves, [topn] of different numbers per each ID

CROSS APPLY (SELECT TOP (topn) N RandomN FROM dbo.TallyN(1000) WHERE n.N > 0 ORDER BY NEWID()) n3

FOR XML PATH('')

), 1,1,'')

--generating ID's - may be coming from a table with actual set of ID's, does not have to be generated

FROM dbo.TallyN(10000) n

ORDER BY id

Execution time - 7 seconds.