Thank you, Peace Train. How much of the smoothness is in the design of the nib versus the lubricating ability of the ink used?
Oh man, that's kind of a tough question to answer. Ink is one thing I haven't messed around with much. I got whatever was suggested to me as being best for a particular pen and
mostly left it at that. I will say that while every aspect of a fountain pen can be as personal and intimate an experience as the words that are penned, the right ink for your pen and nib is as important as any other facet. And the wrong ink may definitely make writing an unbearable experience, if not a chore.
I used to read in forums how people would go crazy experimenting with the right brand, blend, and shade of ink mated to a particular pen. It's one of the reasons I ultimately veered toward new and relatively inexpensive pens from vendors I trusted, and never got into collecting out-of-production rarities or anything like that. There were too many variables I wasn't willing to immerse myself into...along with all the reading in forums lol.
(Although, I did come very close once to buying a rare Japanese holy grail of nibs for nearly four digits, then thought, "Hmm...I might end up in a flashlight forum some day." C'est la vie!). So while I've not had too bad of an experience with ink personally, I'd venture to guess that ink plays up to a 15% role in the smoothness and flow of your writing experience. People in fountain pen forums might argue a higher percent; with a good analogy being like asking in a knife forum, "Which steel is best?"
Just know that anything scratchy is generally due to a poor quality nib, or one that's been misaligned--over time or from being dropped. There are other factors at play of course which directly contribute to smoothness and flow. Weight, balance, form factor, grip, and paper are some. Your writing style is another factor that may affect which pen/nib/ink combo works best for you. For instance, are you looking for something to jot down quick notes, or to relax into the flow--with a snifter of brandy and cigar burning brightly beside you--as you relish in the experience? Is your handwriting neat and precise, or heavy and bold?
Have you ever been in the groove? The perfect pen can do that to you!
(At this point I'm really grateful for auto-save feature on cpf, since the whole page vanished right before I was ready to send. Time for me to go to bed. Hope this helps!)