Is file:memdb making use of an alternate data stream? That would could explain the behavior that you are seeing. It would be interesting to explore the file named file to see if this was the case.

Update: Some other informational links (no assurance of quality on these):

--MidLifeXis

In reply to Re^6: threads::shared seems to kill performance (Workaround). by MidLifeXis
in thread threads::shared seems to kill performance by Jacobs

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