Strange Performance Issue With UPDATE FROM

Hello!

I have this piece of SQL code:

UPDATE a
SET Field1 = c.Field1
FROM a
INNER JOIN b ON a.GUID1 = b.GUID1
INNER JOIN c ON b.GUID2 = c.GUID2
WHERE c.Type = 1
AND @date BETWEEN b.DateFrom AND b.DateTo

This query takes hours to complete.

Now while trying to find out what's causing the poor performance (it
surely looks simple enough!) I've rewritten it to use temp tables:

SELECT a.GUID1, a.Field1, c.Type, b.DateFrom, b.DateTo INTO #temptable
FROM a
INNER JOIN b ON a.GUID1 = b.GUID1
INNER JOIN c ON b.GUID2 = c.GUID2
WHERE c.Type = 1
AND @date BETWEEN b.DateFrom AND b.DateTo

UPDATE a SET Field1 = subsel.Field1
FROM (SELECT * FROM #temptable) AS subsel
WHERE subsel.GUID1 = a.GUID1

Now it completes in 10 seconds.

My question is why? Am I wrong in saying that the two batches above
produce same results? Is there something I've missed about the UPDATE
FROM syntax? Why would the first query perform THAT poorly?

Table sizes:
a: 24k rows
b: 268k rows
c: 260k rows

GUIDs are of type uniqueidentifier.

Any answers appreciated!

Regards,
// Richard