Negligble loss if you use a current sense transformer.
Up to 10A a resistor is quite practical if the sense voltage is low.
You need a current mode controller.
Various ones are upto 50% duty cycle.
Others do the full 100%. Tha'ts what you need.
Look at the data on UC3842 family. They are cheap too.
It will do drive the MOSFET, has current sense, voltage reference, oscilator, comparitor, etc. Very few components needed.
That's one of the easiest way to do it.
If you use hall sensors, staying within the linear range becomes a problem, but you can complicate it by by driving a cancelling coil driven by feedback from the hall sensor. Then the cancelling current is proportional to the measured current.
You can measure right down to DC with response times less than 5uS if it is done correctly. This is really good way to go above 100 Amps.
IMHO it is only worth this trouble above 100 amps.