Cruising speed can also be modulated, easily within some ranges. You might speed up a little if you think you can land cargo before new tariffs, and you may slow if you think tariffs may be reduced if you land later. Staff costs for the vessel are relatively small, so as long as you don't go outside the range of speeds that are fuel efficient, there's flexibility.

Some of the tariff increases are so high that they effectively spoil the cargo; there's no point in bringing it through customs at those prices, and it typically won't make sense to ship it back, and it may not be possible to ship it elsewhere from the port either, so it is most likely to be destroyed at the port. Delaying to see if tariffs go down may be worthwhile for enough of the cargo that it makes sense to slow the whole boat.

Additionally, if demand for shipping is up, going faster allows for more supply, and if demand has slowed, going slower reduces supply.