Ranking is dull. (even in a group it is often rote monotonous activity with no meaningful interaction)
I would agree it that is low engagement and high time investment in today's CF. I don't believe this was always the case though, and I think part of this is actually the lowered challenge to ranking. Soduku isn't as engaging as tic-tac-toe, even though it's sort of the same mechanic, but one of those is challenging and the other is not.
Skill practice is dull.
This is (potentially) high time investment and low engagement as well. The only engagement here is finding an optimized strategy to reduce the time investment, but that's pretty meta and not overly interesting, IMHO.
Gathering preps, gear and gold is beyond boring.
I'd put this in the "Farming" category and agree that it is mostly low engagement and medium time investment.
PK is boring as hell and tedious as a stand alone, I may as well be fighting an NPC with how scripted 90% of fights feel.*
This one is weird to me, as it is the core experience of CF and the most engaging (and lowest time investment) of any activity from my point of view.
Some classes (like paladin) seem to have a low diversity in tactics, and I'd 100% agree they aren't as engaging for PK, but others who have a lot of utility (like Orc, or some warrior builds) don't really follow a script for me.