Whoops, I made a mistake in my last post.

I've been doing a ton of runs lately and I just noticed something about how the final mission time gets calculated into the IGT speedrun time. This may change how you may want to do the final boss in future runs.

For the level PB, this one is kinda obvious. Your mission 20 time is the amount of time it takes for you to climb the stairs and choke the boss out and kill the big demon head, with the timing starting when you can move and ending when [MAIN OBJECTIVE COMPLETED] pops up. Annoyingly enough, this is the time that gets saved whenever you do mission 20 and never changes unless you get a lower time.

For the speedrun timer, the time that gets added to your IGT speedrun time is ONLY the amount of time it takes for you to kill the big demon head. That time starts approximately when the boss's head explodes from inside the boss HQ and the time ends when [MAIN OBJECTIVE COMPLETED] pops up. Why it does this? I have no idea. I guess the stair part is considered too random or something.

You should be able to calculate how much time it takes you by adding up all of your level times (Mission 0 - 19) and subtract it by your speedrun time. The time you get will be approximately the amount of time it takes for you to kill the demon head. I say approximately because the game rounds the level times when they're shown to you on the mission completed screen (most likely by the 4th unseen digit).