An interesting comparison would be a standard cargo ship that goes as slow as possible.

"Slow steaming" is already in use and has some dramatic impacts on fuel use.

The limit seems to be "hotel loads" i.e. constant loads needed for crew, but the sailboat will have those as well and not spread over as much cargo.

It's entirely possible that a sail boat that follows the wind might be higher carbon per cargo kg than a giant standard ship going very slowly directly to the port.