What speed can the inward stellar collapse reach just before core rebound in a core-collapse supernova?

Up to 70,000 kilometers per second.

The gravitational infall characterizing the final moments of a massive star destined for a core-collapse supernova is an extraordinarily rapid event. Once the iron core forms and can no longer support the overlying stellar material by generating outward thermal pressure, the entire structure above collapses inward. This implosion accelerates dramatically due to the increasing gravitational force, achieving phenomenal speeds that can reach up to 70,000 kilometers per second. This rapid compression eventually leads to the core reaching nuclear density, at which point it becomes rigidly incompressible. The infalling material then slams into this dense core, generating the powerful rebound shockwave that defines the subsequent supernova explosion.