Yes, that would mean that the -n 1 case is not working. When size=1, ie., if you have one process, that process is at the same time the first and the last, so I could imagine that the logic in the program is not quite prepared for that and it is trying to read a part of the netcdf file that is out of bounds (i.e, row -1 or row nrow+1 or something like that), and the program abort and does not do any work. That's why it would be fast.
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