People seem to have strong feelings on this issue, so I hesitate to engage. I have no expertise, just a lot of time spent nymphing.

A medium fast rod did fine for me. I tried a much softer and longer Czech nymphing rod and didn't enjoy it as it made picking up the line and heaving it upstream more work. I never tried it with a fast rod, but I don't think that would have made much difference to me. The reason for this is that had my greatest nymphng success on drifts that came downstream within 10' of my feet. There may have been more fish there, but I suspect the biggest reason was that I could achieve a more natural drift close in.