Well, if I understand the pathing system correctly, it tries all least-cost unexplored tiles first(If that tile has, assuming the most optimal path starting at that tile to the destination, the least cost of all other tiles that the pather hasn't explored yet), that using restricted pathing zones where you don't want dwarves going often means that the path must be much longer before it will check that route for a connection, so if it would have taken another route entirely anyway, it will ignore that section for longer, and save cycles.
By putting restricted zoning all over the external areas that dwarves shouldn't access anyway, the pathing would be less likely to spill out into the external areas and flood-fill the map during an attempt to navigate a massive maze.
Of course, if it already does minor pathing on the macro-grid level, such a waste of CPU might be discounted anyway...
Basically, it works like this:
Say we have a fort:
██████████████████████
█++++++█θ+█θ+█θ+█++++█
█+y++++█++█++█++█+x++█
█++++++█+██+██+██++++█
█++++++++++++++++++++█
█++++++██████████++++█
██████████████████████
You want to get from X to Y.
Each square in the fort is labeled with high traffic (1)
██████████████████████
█111111█11█11█11█1111█
█111111█11█11█11█1111█
█111111█1██1██1██1111█
█11111111111111111111█
█111111██████████1111█
██████████████████████
Each 'tick', the path is flooded 1 square outwards.
The numbers and letters represent the number of ticks the pathfinder takes to get to each specific square. (after the 10 digits, I switch to letters, so a=10, b=11, etc.)
It stops when it reaches its destination.
*'s are squares the computer doesn't have to calculate.
██████████████████████
█*gfeee█cc█99█66█1112█
█*gfedd█bb█88█55█1012█
█*gfedc█a██7██4██1112█
█*gfedcba987654322222█
█*gfedc██████████3333█
██████████████████████
Lets say the bedrooms are forbidden (10)
Each forbidden square waits 10 ticks before being checked.
██████████████████████
█*gfeee█**█**█**█1112█
█*gfedd█**█**█**█1012█
█*gfedc█*██g██d██1112█
█*gfedcba987654322222█
█*gfedc██████████3333█
██████████████████████
Because of this, the bedrooms don't need to be checked, saving cycles. However, actually trying to go to a bedroom takes more cycles to calculate a path.