This may be one of the most boring posts but perhaps someone has had a similar problem and found a solution they'd be willing to share.
It takes a full two minutes to drain a quart of liquid out of our galley
sink. All the other sinks on the boat drain very quickly. The drain is connected to a 3/4" hose that has a very short and direct route
to the seacock - there are no kinks in the hose. The sink is at least two feet above the water
line. Out of frustration, I replaced the hose and fittings - no difference. While the hose was disconnected from the seacock, I opened it. A massive geyser of sea water
shot out, confirming my suspicion that the through-hull and seacock are clear.
It's kind of embarrassing to be stymied by something that appears so simple.
Fair winds and calm seas.