It's A Kiddy Dungeon
So apparently it's normal to chant when casting your magic. The more complex the spell is, the more complex the chant would be as well.
I did ask Katsuki why Mary didn't need to chant when she casted her shapeshifting spell back at the mansion.
My Inugami maid just blinked at me and explained that the Head Maid had been quietly chanting while leading us to the gate and the spell was already activated when she stopped,
That was when I remembered that the Head Maid was tier three in assassination too.
I decided that I'll go back and ask Mother about it later.
Since Mary and Sebastian were keeping their distance from us, only Katsuki knew about my lack of chants for now so I asked her to keep it a secret for me, something she swore her life on immediately.