Super basic estimate (I'm not an astronomer, this could be wrong): The core of a star has a radius of 100000 km, and after collapse the radius becomes 10 km. An

Super basic estimate (I'm not an astronomer, this could be wrong): The core of a star has a radius of 100000 km, and after collapse the radius becomes 10 km. Angular momentum is proportional to fr^2, where f is the rotation frequency. Since the radius decreases by 4 orders of magnitude, to keep the same angular momentum the frequency must increase by 8 orders of magnitude. This means that if the new rotation frequency is 1/hour, the original must have been around 10000 years. Statistically some stars like that probably exist somewhere, but it's unlikely that one of the few thousand neutron stars we know of started that way.