Exactly, and these are precisely the vessels that can most handle slower transfer speeds - as you just have more sailboats to make up for slower speeds/smaller sizes. All that matters is how many arrive each day, as the cargo is fungible.

In fact, doing such would provide a buffer that could help prevent supply shocks.

At what point does the additional resource of building a lot more ships (which have more crew, eating more food, producing more waste) result in a net negative?

Oceangoing ships have a relatively short useful life...salt water is rough on metal.

I suppose the upper limit would be when it’s possible to make an ocean-wide winch system and you can just hook barges to it and pull them across the ocean.

Just hook containers to it like cable-cars and move them across.