When HT first came out it was said to provide up to about 30-35% speed increase..

You can see HT as using 'the left-overs' of available computing power that wasnt used by the 1st thread, to execute a 2nd thread...
But the performance gain depends a lot on what these left-overs are, and how well these left-overs are suited to the needs of the calculations in your 2nd thread.

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