Downsampling a PSD that was generated by zero-padding - worth it?
Consider these two cases where you operate on a signal $y[k], K = 0, \ldots K-1$:


You take the DFT of the signal $y[k]$ to obtain a length-K frequency-domain signal.
You first zero-pad $y[k]$ to obtain a longer signal $y_p[k], k = 0, \ldots, 10K-1$. You then take its DFT. Finally, you downsample the DFT by discarding all but the first of every 10 samples (i.e. you keep the first, tenth, twentieth, and so on).


The two processes will yield the exact1 same result.

1 In practice, that is, when using finite-precision arithmetic, it's likely that you won't arrive at results that are identical bit for bit. However, in theory, and with the use of infinite arithmetic precision, the two approaches are equivalent.