That's not even close to true for the kind of large shipping we're talking about. Crews are small (the Ever Given had 25 people on board) but the ships they crew take up to ~100k gallons of fuel per day (Ever Given has a fuel capacity of 3622168.679 gallons, 13711400 liters, and is set up for voyages of ~30 days underway).
Fuel costs are ~$2.5 USD/gallon for bunker fuel. That means a cool $200k per day (conservatively).
It is absolutely not the personnel costs that'd be the big differences in expenses.
Good point, I didn't have a handle on the fuel costs.
Backup argument: if you go at half-speed, you'll need twice as many ships for the same throughput.