1h 38m would be the correct time attack, from start to the credits (fireworks patch)

The runs start the moment players exit the settings screen, and end the moment the unlock screen for DF plays (when the fireworks patch pops up)

The runs have been retimed to fit these criteria.

Though your idea could certainly work, and could provide non-cloud-users a chance to participate too, starting from the chapter itself would make runs into more of a "who can scroll to the next song the fastest" rather than players having to come up with a route beforehand to get to the unlock the fastest. Though it can get narrowed down to one best song route in the end, it will simply be more speedrun-esc and accurate for players to start from the very beginning.