Can't help with the physics, but in addition to more variable times and potentially different textures that have been mentioned, the other convenience of boilin

Can't help with the physics, but in addition to more variable times and potentially different textures that have been mentioned, the other convenience of boiling water first is that it can be covered without causing trouble (potentially saving some energy), while lots of stuff (including pasta) can't unless you watch them. Pressure cookers always start from cold, although the pressure and the tighter seal change some considerations. Not related to the question but buckwheat is a good pasta alternative, quick cooking and goes great with pasta sauce; should be much lower total energy use, particularly with untoasted buckwheat.