Personally I think it is more about efficiency than speed. Of course when you are efficient you can also go fast.
The number of calories expended has a lot to do with your efficiency. Meaning if you are wasting energy by slipping and sliding (the wrong way) then you wll use more energy. For instance a good skier on a groomed or tracked out trail will travel much more efficiently than someone postholing. When covering terrain that results in skiing inefficiency such as a trail with many feeder stream ravines, steep hills requiring herringboning, or even flat sooth trail covered with slick ice that resists all but the most sticky klister....other forms foot travel will be more efficient. So, for me, if I use snowshoes and tracking aids, I can usually cover mountain trails more efficently and also quickly than on skis.
Calories also includes the amount of trail you have to break. Skis have a narrower footprint. It gets more complicated as you have to consider how much you compress vs displace. This is less of a factor for me as I prefer to follow a broken trail
Calories is also is effected by the mass you need to move. Consider your both body and your feet. Each meter of distance requires lifting feet with their associated weight (boots, showshoes.) Comparing the weight of the ski boots and skis vs hiking boots and showshoes. Skis are the clear winner here vs snowshoes. But keep in mind that the best ski conditions are just awesom when cranking with lighter traction aids. Also, your foot travel rythem includes an up/down motion of your body. I think here skis also win. And lastly there is the elevation gain. Overall pack and body weight are probably comparable.
For me, I often say I can cover mtn trails faster and more efficiently than skiers because these trails often have sections that are not conducive to efficient skiing, some of the people I hike/ski with are not superior skiers, and I have spent so much time trying to improve my snowshoe and winter walking technique.
Of couse I am only human and I my technique is nothing compared to the rabbit PinPin Jr or 3.
One testimonial: I was on an AMC trip to the Captain. It was a group of skiers and hikers. The skiers swithched to snowshoes after the apprach on Sawyer Pond trail from the Kanc. On the return trip, when we got down to the trail, the leader said, "let the skiers go first because they are faster." All the skiers took off. I was in 8 point crampons for traction on hard pack trail. I walked along and sequentially caught up to and briefly talked with each skier before passing them. When I caught up to the first skier, I was following right behind her at about 3.5 mph pace. At the final (big) river crossing I continued accoss while she removed her skis for the crossing.....antd that's how I met Sue Johnston.