what video problem will be introduced when frame rate is dropped?

Again, don’t try dropping the frame rate from 29.97 to 15, because you can introduce more problems than you solve.

( what video problem will be introduced when frame rate is dropped? )

When FPS drops below 30fps, the human eye can start to detect that the 3D image starts to appear jerky and "laggy"

time and control codes

timecode is physically recorded with the programme, its relationship to that programme is fixed, and can be used as an accurate reference.