problem with a simple buck converter
Why are you using an N-channel MOSFET as a high-side switch? That's where your efficiency is going &mdash; into excessive drop across the MOSFET.

The IRFH7921 is intended to be used as the synchronous rectifier (i.e., the low-side switch) in a synchronous point-of-load converter. You need a P-channel MOSFET for the high-side switch.