Yup. CZs is the only place I've seen my rank go up, but even then progress is slow. Granted I only did low-CZs, but I would have expected clearing a handful (<6) of low CZs should have pumped me up to Mostly Defenceless ->Rookie, given the same amount of Low-CZs in a ship would achieve a comparable result for Elite rank.Did you do CZs?
It goes up with every kill, I can confirm that. But it only goes up "significantly" if you do CZs. High CZs will move it at the sort of rate you'd conisder similar to combat ranking for ships (although still not nearly as fast); the way ship kills work is you get exp for every kill (irrespective of the target) but you get more exp for a kill depending on the target's rank. It's as though Fdev made high intensity CZ targets "elite" and everything else "harmless", in terms of how much you get for each kill. I say that but it's still not quite true: killing just harmless ships will get you to "mostly harmless" after maybe 5-6 kills. Not so with mercenary rank.
It's pretty easy to recreate the issue, too. Start a new commander and spend 3 weeks killing just scavengers and you'll not hit "mostly defenseless" at all. Spend a few hours doing just high intensity CZs and you'll rank up pretty fast.
The only conclusion I can make from this is that they've messed up the way we get exp towards ranking up.
As a contrast, a single Exobiologist discovery got me 1/3rd of the way to the next rank. I'd put that on-par with clearing a low-CZ (if not clearing a low CZ is more effort than a single scan). With a dozen low-CZs under my belt it still took me a month or two of doing massacre missions/assasinations to get Mostly Defenceless. That can't be right.
As another contrast, my new account hit Mostly Aimless/Mostly Harmless/Mostly Penniless in a day.