Does it come as a big surprise that someone who has played this game for years (and says he plays 5-8 hours per day) and is probably far better than average can grind up a line very quickly? That's how virtually all of the online "FTP" games I've played over the years work, if that player pays attention to learning some mechanics and improving his play along the way.
I started playing about 1 1/2 months ago have less than 600 battles (random and coop combined). For me that's a lot of playing (and will slow down soon due to RL commitments). I will probably unlock my first tier 7 ship the next time I play. My best captains are a couple 8 pointers. I don't think this is too fast. but it's probably slower than average. Unlike the OP, I have been grinding several lines at once to try different boats and play styles. I haven't paid for premium time or higher tier ships. You don't get many signals or camo's in the lowest tiers. I think it's around tier 4 or 5 that you can start getting them more regularly from missions and campaigns.
I was able to progress through the first 3 or 4 tiers fairly quickly, but then it starts slowing down. It seems like going tier 6 to 7 probably took as long as going from 1 to 6. I anticipate each tier will be progressively slower for me but based on what I've read I'm content to stay around this tier for a while. I have made a small purchase to support the game but not for things that significantly speed up the grind.
So Flambass can grind a line in 6 hours with all of his advantages (knowledge and skill perhaps being his biggest advantage) and on the other extreme a new player who doesn't pay to accelerate up the tiers has only made tier 7 in about 6 weeks. I don't think progress is too fast unless you're paying to bypass the grind, and jumping ahead. I don't think it's the speed of the grind that bothers most of you, it's the ability to pay to bypass it. But the game has to be profitable and I don't see how you avoid that problem.