Roughly one hit in three stops your victim drinking for 1 second. At most one block every 2.5 seconds, whatever your click speed.
On a soup server, soup is the health bar. Blocking healing is not "a bit more damage": it changes the nature of the fight.
The randomness is deliberate. A block on every hit made the victim unable to drink at all, with no counter. Making it unpredictable means they can no longer count hits to know when to risk a sip — but they know they will drink within three seconds at worst.
The counter-play is to back off: the salt only renews on contact. A Shio who stops hitting loses the effect in a second. The kit forces constant aggression, which is what exposes it.
Melee only — a Shio with a bow could cut healing from forty blocks away without ever exposing himself.