Mikael,
you should post your code as code and not an image.
I would do the numerical before the Im[] but it is only a minor speedup. This will help:
AbsoluteTiming[ParallelSum[Im[N[ZetaZero[i]]], {i, 1, 10000}]]
it takes 32 seconds on my machine with 8 kernels which means you would need 100 times that to get to a million or 3200 seconds (about an hour)...
Regards,
Neil
