How quickly does the iron core shrink from Earth size to 20 kilometers across?

In a matter of milliseconds.

The gravitational collapse that follows the formation of the iron core is an event of extreme rapidity, contrasting sharply with the eons or millions of years the star spent in stability. Once the core mass surpasses the critical stability threshold, gravity completely overwhelms all remaining internal resistance, initiating a catastrophic implosion. This collapse is so violent and swift that the core shrinks from an initial size comparable to that of the Earth down to a dense sphere only about 20 kilometers in diameter in a period measured in milliseconds. This instantaneous physical transformation is the foundation for the subsequent core bounce and shockwave generation that leads to the visible supernova.