Lock stiffness can come from many things. In my experience, the hardest locks I’ve ever had were that way because of buildup. Lint and dirt packed into the cores of your locks definitely make your locks rather heavy and stiff. However, what make them heaviest and stiffest is product buildup. I had a lot of this, and the only way to get it out was by brushing those locks out.
Of course, a deep clean is a good thing to try in the event you suspect you have product buildup, but if that doesn’t work, then brushing your locks out is most likely the only ay to get rid of it.
Otherwise, your locks could just be very mature! Some people’s locks get harder than others, just because their knots are tighter. Since you said this is characteristic of your older locks, I suspect this is the issue, so don’t worry!
If it bothers you that much, you can try conditioning them, but it probably won’t do much. Everything sounds fairly normal to me!
Followers: Any suggestions for how to make mature locks softer?