What is the fastest spinning type of star?

A millisecond pulsar - a neutron star rotating up to several hundred times a second, so a body more massive than the Sun and about twenty kilometres across turns faster than a kitchen blender. The fastest known does about 716 rotations per second, which puts its equator at a fair fraction of the speed of light.

They spin that fast because they were spun up. A neutron star that has slowed over millions of years can be accelerated again by stealing matter from a companion star, which arrives with angular momentum and acts like a hand on a flywheel - so millisecond pulsars are old objects recycled, and are usually found in binaries.

Their timekeeping is extraordinary. The pulses are regular enough to rival atomic clocks, which is what let the first exoplanets be detected - around a pulsar, in 1992, three years before the first one round an ordinary star - because a planet's pull produces tiny timing shifts. Arrays of them are now used as a galaxy-sized gravitational wave detector, watching for correlated deviations across many pulsars at once, which reported its first evidence in 2023.