I didn't read the novel, but I agree the pacing of the manga is incredibly irratic. It's like it doesn't know what it wants to focus on, and the story doesn't seem to really go anywhere.
I mean, the school arc is just exhausting to go through. Events have no emotional depth to them, since we already know the result beforehand, and any consequence is nill.
Shit just happens and repeats itself with no real purpose, yet the plot also advances in the timeline really quickly, as if nothing matters.
TLDR: Art is very good, good start, real mediocre ten chapters in.