The Slowing attack, known as AD_SLOW in the source, is an attack possessed by shades and skeletons. It removes both intrinsic speed and temporary extrinsic speed (from the spell or potion) if you have either or both. Your dexterity is abused. It does not affect permanent extrinsic speed, as provided by speed boots. You will still lose both intrinsic speed and temporary extrinsic speed. Slowing is successful only 1 in 4 times the attack hits, and is additionally affected by magic cancellation.