Glassbox engine has been shown as a challenging and CPU-sucking way of solving the simulation. I'd love for it to actually work, but is it really so difficult/r

Glassbox engine has been shown as a challenging and CPU-sucking way of solving the simulation. I'd love for it to actually work, but is it really so difficult/resource intensive to solve the problem? In old SimCities one could click a house and see which route that invididual worker would take, e.g. a (permanent?) mapping between worker and job, student and school, etc. Of course, then the actual traffic simulation in previous SimCities was just crap. Why couldn't an agent based system work, just that you keep in memory for each agent where they belong and where they go? And then at regular intervals (every few game days or so), randomize some of those mappings to create people moving, jobs changing, etc. Is it that the path-finder is too resource demanding if every agent has a pre-defined destination, and much less taxing if just picking random targets?