I have just tried this in the Preview of the Schema (to remove the API from the equation) and with only 20% of the dataset loaded, it just shows the generic "Something went wrong" message after 5 minutes, when I am only asking it to generate 1 row.
So there's something definitely running slowly in the from_dataset function here, as if I remove the formula and just use a constant value, it returns instantly.
Any thoughts @mockaroo?