A ship going through water has hull drag which shows up as a bow wave. This drag is a (decreasing!) function of hull length (sqrt(LWL)). Longer boats have less drag. What the bulbous bow does is it creates a counter bow wave which further reduces this drag. However this magic only works at a specific speed tuned for this hull.
On the San Francisco Bay, for example, a container ship will leave the container port (Port of Oakland) at a slow restricted speed (5 knots) inside the Bay. But once she exits the Bay and the shipping channel and hits the open ocean, she will increase her speed to maybe 25 knots to hit that tuned speed with less hull drag and efficiently cruise across the Pacific.
However, the shipping industry noticed that no one wanted to pay for that speed. Enter Slow Steaming. Boats could be retuned for a slower but more fuel efficient speed, maybe 20 knots. The industry literally took existing ships into dry dock and re-nosed them. This meant transits took longer but also used less bunker fuel, the cheapest and worst fuel which is only allowed to be used offshore. This both saved money and reduced pollution.