Cast iron corrodes from the inside as waste and moisture pass through it for decades. Scale builds up along the pipe wall, narrowing the usable diameter and slowing drainage generally, not just at one clog point, while the corrosion eventually pits through the pipe wall itself, causing leaks at low points or complete failure at joints. Because the narrowing happens gradually across the whole run, a home with cast iron often has multiple slow drains at once rather than one obvious problem.
The material was common in construction through roughly the 1970s, in some markets a little later, which is well before most of Weston was built. Where it does turn up here, it’s usually in a section that predates the main development wave, or in fixtures salvaged or connected from an older structure, and we confirm it with a camera before recommending anything.