I get one Fenix LD22 from its website https://www.fenixlighting.com/product/ld22-fenix-flashlight/
It has pretty solid design and specifically optimized for AA size Alkaline or Ni-Mh battery and I think it is designed smartly.
Alkaline or Ni-Mh battery's normal working voltage range is 1.0 to 1.5V. So with 2x AA size batteries, the voltage supply of the led is actually 2.0 to 3.0.
It uses a Cree XP-G2 LED bulb with near 3.0V nominal voltage. Cree XP-G2 outputs 150 lumens with 350mA current drain and 300 lumens with 750mA current drain.
The flashlight may use a booster convert to boost 2.0V~3.0V to 3V. The convert may have a decent efficiency near this range.
At 300 lumens mode, the converter would draw more than 750mA from battery. Alkaline battery works terribly in this mode due to its discharging curve. Quoted from http://lygte-info.dk/review/batteries2012/CommonAAcomparator.php
With 1A current drain, Alkaline works much worse than Ni-Mh.
At 100 lumens mode, the converter would draw more than 350mA. This is actually sweet region for Alkaline batt. Alkaline outperforms Ni-Mh for current drain below 300mA.
LD22 reaches a decent balance between Alkaline and Ni-Mh. AA size battery can be found everywhere and Alkaline delivers decent performances below 300 lumens mode. Ni-Mh battery covers pretty every mode and is favored in the manual. Eneloop is a perfect match for LD22. 2x count outputs good efficiency.