How dramatically can the rotation speed of a newly formed Neutron Star increase compared to its progenitor star?

It can spin hundreds of times every second

As the massive stellar core undergoes catastrophic collapse to form the tiny neutron star, the physical principle of the conservation of angular momentum comes into sharp effect. This principle requires that as the radius decreases dramatically—squeezing mass into a city-sized object—the rotational velocity must increase correspondingly to conserve the total angular momentum, analogous to an ice skater pulling their arms inward. While the original, large parent star might have rotated slowly, perhaps once every several weeks or months, the resulting compact neutron star can achieve astonishing spin rates, capable of completing hundreds of rotations every single second.