Spiral staircases placed incorrectly can cause lag and routing issues. For instance, there's one particular lot in Isla Paradiso that is particularly responsible for grinding that town to a halt, because the bottom step is turned into the wall.
Another problem are public markers not placed in elevator shafts. I never played Bridgeport long enough to know whether this is the case for all the lag issues there, but I discovered the problem in one of my own worlds when sims weren't able to navigate to the second floor and were stranded in the lobby.
@Turjan is absolutely correct about too narrow spaces causing lag. This issue is part of the reasons why Isla Paradiso is so laggy and has serious routing problems. Too many bathrooms and bedrooms are only 2x2 tiles wide, which is a recipe for disaster.
