I thought the pump repeaters were pretty straitforward; moreso then the floodgate one.
That said, they aren't going to be easier to build.
Also I was under the impression that it only recalculated paths when a dwarf bumped into an obstruction, so unless you're routing your dwarves through the repeater, it shouldn't cause pathfinding lag--and even then it'd be minimal.
There may be other sorts of lag, though.
Yeah, we're talking about two different things. The game first builds this sort of map to say what tiles are somehow connected, like how the third cavern does not originally include a path to above ground. This stops creatures from trying to go to nonsensical destinations and also keeps all of the solid rock out of the equation. This obviously needs some computing when there is suddenly a new wall somewhere or suddenly is not one.
Within those spaces individuals will set destinations and then check for the shortest path they can take. When they bump something they then retry with simple pathing (ignores 4/7 water, traffic designations, maybe other stuff) which is what you were thinking of. This is kind of a resource hog but it is much smaller than it could be thanks to the pathing maps built by that other thing. Unfortunately simple pathfinding doesn't immediately check if the destination is still in a connected zone which is why you get such nasty lag when you build a wall to keep impending doom away from the fort. After like 100 failures to path a unit gives up though so we don't see them swarm as soon as a wall is taken back down.
One day Toady will change some aspects of this but right now it makes sense in most circumstances and he will probably never do full AI for swimming, flying, and building destroyers cooperating or anything like that. (Can you just imagine how complicated it would be trying to decide the shortest path when you had to consider when some unit was going to bust open a door or when something was going to swim into some place and pull a lever to open a path? Sure it's POSSIBLE to make the computer work that out but not in a way that would run at a bearable speed.)