You don't need 256 bits of entropy, you only need 128.

I have tested this method on over 100 different CPUs and I have never seen such consistent output. I'm genuinely surprised to see that you only hit 92 bits of entropy, but that can trivially be fixed by doing 10x the iterations. 500 iterations is still going to put you under a millisecond of cost.

And, for what it's worth, code I've actually shipped has combined the above technique with Fortuna, and has typically targeted 2000 bits of entropy rather than 128 (for security buffer).

EDIT: I reviewed his code, and he's not hashing between calls to check the clock; the hash call itself causes the CPU to heat up in arbitrary ways which changes the timing between hashes and introduces more entropy; removing that call basically entirely defeats the idea behind the technique, these results are fully invalid.

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I updated the code to insert the hash call, this is what I got for his original code on my machine, and the updated code with hashing on my machine (and the difference is cryptographically meaningful):

  === Original C — no hashing ===
  Clock resolution: 0.000000001
  Deltas (ns):   50   34   19   19   13   13   13   13   13   14   13   13   13   13   13   14   13   13   14   12   13   14   13   13   13   14   13   13   14   12   13   14   13   13   14   12   13   14   13   14   13   12   13   14   14   13   13   13   13
  Deltas of deltas:   -16  -15    0   -6    0    0    0    0    1   -1    0    0    0    0    1   -1    0    1   -2    1    1   -1    0    0    1   -1    0    1   -2    1    1   -1    0    1   -2    1    1   -1    1   -1   -1    1    1    0   -1    0    0    0
  Maximum entropy: 90

  === C with SHA-256 between clock reads ===
  Clock resolution: 0.000000001
  Deltas (ns): 756852 1287  542  470  472  445  442  436  434  439  488  435  433  434  440  439  439  435  432  433  435  432  433  433  429  433  453  441  437  437  431  433  432  430  431  438  436  434  431  433  435  436  435  433  430  436  435  437  428
  Deltas of deltas:  -755565 -745  -72    2  -27   -3   -6   -2    5   49  -53   -2    1    6   -1    0   -4   -3    1    2   -3    1    0   -4    4   20  -12   -4    0   -6    2   -1   -2    1    7   -2   -2   -3    2    2    1   -1   -2   -3    6   -1    2   -9
  Maximum entropy: 188