Yes, but slow-steaming for efficiency on a cargo ship is ~18 knots, which is still above the upper speed of the fastest sustained speed of sail cargo vessels. You can slow further for efficiency, but you hit a tipping point pretty soon where you end up costing yourself more in time than you save in fuel.

Realistically, a sail cargo ship is going to move along at a rate less than 10 knots.

And of course this is all on a route with favorable winds. If the winds are against you, all bets are off.

The same is true of these wingsails, there are limited routes where they will be efficient. Anything going up-wind is actually going to be a loss.