It's actually "slow goblin's curse" and slow is describing the goblin and not the curse. Otherwise it'd be "goblin's slow curse." Plus curses are commonly permanent. Why would it be slow?
by that last bit of logic, if they were curses then food poisoning (which sets in quickly) should last forever, while lung cancer (might take a long time to be discovered) should fade away in a day... right? because permanent things have to happen quickly? obviously the onset speed of a condition has nothing to do with its permanence, whether it's magical in nature or not lol. and i'd imagine something as stereotypically weak as a goblin doesn't have the magical capacity to snap its fingers and convert someone of a different species' biology to be able to bear goblin children in a day, you know? hence it being a slow curse. maybe they can cast a relatively simple curse to kill someone or whatever & that takes effect within the day, but this is clearly more complicated and energy-consuming than just stopping a heart. they cast it on the prisoners, feed it magic over a few weeks or months, and eventually the victim is converted to be able to make more goblins.
a curse by a goblin.
hence, a goblin's curse.
but it's a slow-acting curse.
hence, a slow goblin's curse.
that is to say, [slow [goblin's curse]], not [[slow goblin]'s curse]. like where are you getting this absolute certainty that "slow" is referring to the goblin who cast the curse and not to the concept of a goblin's curse that takes hold slowly lol
edit: last sentence comes off wrong, not trying to sound like i'm insulting you since you're not wrong in your logic, just your... reasoning? or vice-versa? like technically you're absolutely right, the descriptor "slow"
can easily be referring to just the goblin, but that doesn't mean it can
only refer to just the goblin. and logically since they're talking about a curse (cast upon them by a goblin!) which slowly and torturously transforms them, it's a bit odd to interpret that as them suddenly dropping the name of a new breed of goblin that nobody has ever heard of before or since, right? the certainty behind your interpretation feels like it's being read as a proper name (Slow Goblin's Curse), which just feels like the name of a hipster restaurant or something, lol. and in that situation
I'd be the one sounding like i'm deliberately misenterpreting things if i insisted that "Fat Jim's Buffet" had to mean "Jim's buffet, but the walls are filled with cholesterol" instead of your correct inference that it's just a buffet run by some guy named Jim, who happens to be notably overweight compared to most other Jims you'd meet in the wild...