This has been mentioned before a fair amount prior to 1.0 release, but now that we've got the full game I think it still needs addressing.
The jump from sec 0 > 1 > 2 is very quick, which is reasonable as that's when the game opens up. At 2 you have low level guns, still the terrible backpack and vest, and are generally limited in stims and food. This takes a reasonable amount of time to get to 3, which opens up some other options, but you still don't get any ARs (excluding stashed and the ak74u iirc), meaning you're reliant on the mosin you find and later the AK the game gives you.
3 to 4 felt like a slog. Rec centre, then the diving deep mission, and finally the outskirts mission feel like a really long haul to get to 4, just to give you the option of buying guns at that level, when realistically you've been using them for a while from stashed finds.
So after having run an AR or at least the mosin for all of sec 3, you're rewarded with... more ARs and the potential to run suppressors + NVDs with a helmet. The change here felt like almost nothing to me personally, except I now had access to outskirts and better rewards from missions.
So after a long wait for sec 4 for not much of a gameplay benefit, you then get sec 5 quickly and just before the final mission, and are given very little opportunity to use the guns in that tier, bar the Groza if you count it as sec 5.
I think mission 6 should give sec 4. I understand the dev vision that you go into outskirts, do the mission and then get sec 4, but it pushes sec 5 so far back that we never get to play with the guns. They're the best guns in the game. Leaving them to the last second so I only get a couple of hours with them is terrible.
Tl;dr, sec 0 through 3 felt fine, maybe 2 was slightly too long, but 3 > 4 took too long for no real change in my loadout, and 5 > ending was too short. I would rather have a very short 4 > 5 for more time to play with the best guns, especially when 3 > 4 gives very little.