I was working on something and I decided to use something very similar to that (check the image file), and it relied on locking repeaters. I would have two lines of repeaters (I had them at 4 ticks) and I would unlock them, at the press of a button (for 4 ticks) to let the signal propagate (so that I could have, for example, a lock, where you had to solve it once, let the signal get to the next level, and then put in the next combination). Problem is, (at least with the setup I used,) if I turned off both levers, pushed the button, and then turned on both of them, I noticed that the left signal would travel faster? (I'm not sure, it just gets to the next row of repeaters when it shouldn't). And this only happens to the left (furthest from the power source) line. I'm saying left, because I build it a few more times with different orientations and it always happened to the left line.