Fastest mollusks?  Squid, far and away.  So fast that when they are swimming
toward the surface, especially when being pursued by predators, they may leap
out of the water and soar 30 to 50 feet, perhaps more.  I almost got nailed
by one once when I was on a ship.  The thing slammed into the bulkhead
(that's a wall, for you landlubbers) about ten feet from me.
Fastest gastropods?  Seahares (family Aplysiidae) get my vote.  Some of them
can outswim a diver with fins.
Fastest bivalves?  Pectens probably.  When they clap their two shell valves
together they can zigzag through the water at a pretty good clip, though I
never saw one that could outswim me.
Paul M.