Thank you so much for playing and for the detailed review!
The speed of the monsters does change between levels, but not during their approach. It's linear, however, their scaling up might give you the impression that they slow down when getting close. That's a great idea though.
Lvls 11 and 12 are especially difficult because we wanted players to have that great accomplishment feeling, similar to when you beat a Dark Souls boss. Don't feel bad about it though, even us devs of the game have a hard time beating it lol