⟨12sichelgaita⟩ Largely off-topic, but here are the benchmarks on my machine.  The number after "Cpp/" is the degree of the geopotential:
Run on (48 X 3793 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x24)
  L1 Instruction 32 KiB (x24)
  L2 Unified 512 KiB (x24)
  L3 Unified 32768 KiB (x4)
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Benchmark                                       Time             CPU   Iterations
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BM_ComputeGeopotentialCpp/2                   100 us          100 us         6400
BM_ComputeGeopotentialCpp/3                   124 us          122 us         4978
BM_ComputeGeopotentialCpp/5                   190 us          188 us         3733
BM_ComputeGeopotentialCpp/10                  557 us          558 us         1120
BM_ComputeGeopotentialDistance/150000         102 us          103 us         6400
BM_ComputeGeopotentialDistance/500000        42.0 us         42.4 us        16593
BM_ComputeGeopotentialDistance/5000000       8.96 us         8.89 us        89600