I would say its definitely both. Your GM is relatively inexperienced with the system and therefore refers to the book or other databases when it comes to introducing monsters. You probably haven't modified or made up any new classes yet, either (f.e. a thief that doesn't have poisoner but instead goes poof and stealths :) ). This, of course, means that things will slow down a bit.
Since it sounds like you aren't the GM, encourage them that they have the trust of the group to make monsters up on the fly.
The group enters an old ruin with stone statues all aorund... the thief, suspicious, looks for traps - oh yeah, the statues are the traps and suddenly, some of them come to life!
Statues naturally will have high armor, maybe they can't even be damaged by mundane weapons because, well, stone. Magic would work, maybe the wizard enhanced the groups' weapons with magic before this if they were prepared? :D
Huge stonestatues would also naturally have a knockback as an attack-effect, they would just try to corner and stomp a person basically. No piercing damage, though a hard move could be damaging some metal armor, certainly. The statues could have around 15hp if its a group, about twice that if its just one huge golem.
And thats how you make up a thing on the fly. Think about the fiction, what would make sense for a stone golem to have - they are strong & durable (4 armor maybe?) and they want to protect the area, so running would f.e. totally work in this scenario, they wouldn't give chase.
It might help your GM to think of a theme for the dungeon and adhere to that. A cave infested by goblins will have different traps and other threats than a tunnel system made by giant worms.
To summarise: Given time, experience and especially trust from the group, a GM can and (in my opinion definitely) should make up creatures on the fly... it makes for more interesting play anyway, as I doubt that the book or a database would have stats for, say, Birds that are actually part of a tree creature and 'gather' people onto the tree spikes so that the tree can feed on the blood of the victims... (I go for weird - a lot).