... calculate the standard deviation of the values of a small slice of the initial array ...

If it takes longer to spawn the thread, than to process that small slice, you have a net slowdown.

How long does a single (non-threaded) solution take? How many cores do you have? How many concurrent threads does that graph represent?

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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In the absence of evidence, opinion is indistinguishable from prejudice.
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In reply to Re: Poor performances with threads by BrowserUk
in thread Poor performances with threads by olafmar

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