How to slow down a program and why it can be useful

The authors have tested a rather obsolete CPU, with a 10-year-old Skylake microarchitecture, but more recent Intel/AMD CPUs have special optimizations for both NOP and MOV, executing them at the renaming stage, well before the normal execution units, so they may appear to have been executed in zero time.

For slowing down, one could use something really slow, like integer division. If that would interfere with the desired register usage, other reliable choices would be add with carry or perhaps complement carry flag. If it is not desired to modify the flags, one can use a RORX instruction for multiple bit rotation (available since Haswell, but not in older Atom CPUs), or one could execute BSWAP (available since 1989, therefore it exists in all 64-bit CPUs, including any Atom).

The idea is that these are unavoidably slow and preferably non-parallelizable to compute one way, but fast or near instantaneous to verify the result. Examples include the Weslowski VDFs based on similar math to RSA, MIMC, and the use of zero knowledge proofs to provide proof of slow computations.