I don't have a lot of experience with Czech lines, but I've known many high % DDR dogs. There was a pure DDR stud in the area for many years that got used a LOT by anyone and everyone, but I'm not basing my opinion just on him as there are also a couple other breeders in the area who focus on pure/high % DDR dogs, and quite a few of their dogs around. I do find them slow to mature, not physically but mentally.
Many have been sleepers, showing not much more than pet temperament until 1-2 years old, and then suddenly waking up.
With drive, there often isn't much there when young and it takes a long time to come out.
The puppy "attention span of a gnat" phenomenon sticks around longer, and it can take these dogs longer to learn to concentrate, resist distraction, and focus on the task at hand.
Overall just goofy, sunny, somewhat flighty personalities typical of young dogs. They take a while to develop seriousness, dignity, calmness, purpose and work ethic.
Many I've seen are quite handler soft as pups, and many don't grow out of that.
Suspicion and defense often develops earlier in these dogs, as opposed to western lines, so in that sense they can be quicker to mature. BUT, it also often comes about before the dog's have the mental maturity, confidence, and life experiences to know how to deal with those feelings, which can make them prone to overreaction. And that in turn can lead to some real squirrely behavior and reactions that might indicate weak nerves when young, but it evens out and the overreactions can go away once the dogs mature more.
Of course, this slow maturing isn't just DDR lines. Dogs from other lines can be slow to mature too. And some DDR dogs mature at quicker rates. Just speaking in generalities, but slowness to mature is more prevalent in the DDR lines than others.