I got these 94 duro aftermarket Thunder bushings in that sick teal blue ish color. Do Thunder bushings get a bit stiffer over time?
Bushings are urethane so have two things that usually go on.
As they age, they firm up a little, as per all urethane, so from the first day of use, gentle break in period to pretty much about usual after a week or so, they will feel a bit firmer or at least bounce back, rather than stay in the same position you landed.
If they are under pressure they will compress some as well, so as the bushings are squashed, they will make the trucks feel a little more loose to some degree.
A friend recently got those exact ice blue 94 duro bushings and at first they were quite stiff and non responsive, but now after maybe a month or so, they are more responsive but don't feel quite as difficult to turn with but he did tighten down the kingpin nut about half a turn at the last session on them.
They are way stiffer than the stock bushings that came in the trucks though, even though with the same wear in period, they felt good to me, he wanted them a bit stiffer, so these work well.