IMO - that would depend on which ‘regular’ low D you compared them to, as low D’s themselves also vary in loudness and amount of breath needed.
Yes I don’t know if there’s such a thing as a “regular Low D whistle”. I’ve owned Low Ds from most of the leading makers, including multiple examples from some makers (five MKs, two Burkes, three Overtons) and not even the whistles from the same maker are the same.
I did play Kena (G) and Kenakena (low D) quite a bit around 20 years ago, and I’ve dabbled with using the Kenakena in Irish music.
What I did, 20 years or so ago, was to make a couple of these Low D whistle/Kenakena hybrids out of PVC pipe. I carved out the notch by closely following the pattern of my fine-playing Kenakena. No it doesn’t take a lot of air! By using a fairly narrow notch and with a good embouchure it can be just as air-efficient as any Irish flute. Then I chopped the bottom to made an in-tune Bottom D, then drilled the fingerholes following the layout of an Irish Low D whistle. The results played nicely. What’s cool is that you have the flexibility in timbre and volume that you have on the flute (you can play any note at any volume you want) but with a vertically-held thing.
I did get to try these Open Whistles during the tour and I thought they were OK. Honestly my trusty old Kenakena plays better. (Those guys in Bolivia have thousands of years of experience making and playing those!)
Here’s how nice a Quenacho can sound! And later he plays the tiny Quenillo