Thanks for the suggestions. I do all the other stuff (low quality, spam the button, position mouse in advance etc.).

Currently I use

Alpinia (700 Spd, + 150 Squad Spd)
Anthurium (not evolved as only level 32, 700 Spd)
Edelweiss (515 Spd, + 50 Squad Spd)
Setaria (720 Spd)
Gaillardia (560 Spd, + 50 Squad Spd)

My squad speed is 889.

I take out Anthurium or Setaria and my squad speed drops.

Dipladenia is only level 32 at the moment too. I will try go evolve one of Anthurium/Dipladenia for next time!

Well well. Too much girls in your team. If your Alpinia is evolved/max affection and with an enough good level, use only her and Setaria.
If it can help you, I can do the boss in one turn with only Anthurium/Dipladenia max affection, lvl70. Also, prefer allies with high speed team. It will take less time to auto-reach the end after your team reach it first.

Just try. If it's not enough to kill the boss, add Dipladenia or Anthurium. You will have enough speed with them I think. Maybe try Alpinia alone. She is a 6* after all. At high level she can certainly kill the boss alone.

Just remember, the more girls you have, the more attack/buffs/skills animations you have. And all this stuff take too much time.
If it's possible, avoid girls with loooong skill animation like Dipladenia. Oncidium/Setaria are good for that.
In the future, having a lvl1 skill Dipladenia just for that can be a good idea since she don't have any buff at the fight start.

And... that's all I think. Pray for not too long loading and no damn Loading Error.