I've recently found that on relatively "slow" dives, I do alright. My first dive back in the water after >3 years, which is only my third dive in over a decade, I was <0.5cuft/min, and came out of the water with 750psi left in an Al80. The two much more experienced and active divers I was with, both petite women, were only 2-300psi ahead of me with Al63’s. I didn’t do the math to compare their consumptions and mine, clearly they were much better than mine, but I was shocked and excited that my first dive back didn’t wind up with me thumbing the dive because I sucked through my air long before anyone else. This was shallow, mellow, not a lot of ground covered. Spent a fair amount of time hanging out with fishes and anemones up close.
Next dive was relatively deep (115ft), with an instructor, a divemaster, and two other very active divers. All guys, all but one in excellent physical condition, and all much more aggressive than my two previous buddies. We swam at a moderate pace and covered a pretty fair amount of ground, and I absolutely HAMMERED my gas trying to hang with them. I let them all know I was low, and popped ~20ft above them to start my safety stop earlier than the rest. I hit the surface right on 500psi, while most of them were still 8-900+ on the same tanks, 2-3 minutes behind me.
I absolutely believe comfort in the water, and conservation of motion will help a lot, and every one of my buddies had those traits far more in hand than I currently do. However, I also believe fitness is critical. I’m extremely average in terms of fitness. Lots of room for improvement, but I’m “fit” enough to be healthy from a medical perspective – cholesterol, blood pressure, spirometry, etc. are all better than average, year after year. The reason fitness stood out to me, was the one “less fit” diver on my last dive was notably hanging out ~10ft shallower than anyone else for the entire duration of the dive, sometimes much shallower. On surfacing, he was right in the pack of average for pressure left in the tank – still ahead of me by a couple hundred PSI, but behind a couple of the others, and at no point prior to our safety stop did I see anyone within 10ft of his depth. If he spends the duration of the dive at 1/3ATM less than anyone else, is far more experienced than I am, and he certainly looked plenty relaxed and calm in the water…all I can come down to from there is general fitness.
Also, if there's no need to go deep, don't. Easiest way to improve your gas consumption is stay shallower. I was nose into the bottom, seeing the sights for most of our dive. That certainly didn't help my case.