I think maybe that's a good subject for a new thread! While I ponder what to include, in what is indeed a quite complex topic, let's start with basic categories of reasons:
1) "Cracking effort"
2) Dynamic Flow
a) Venturi effects
b) Case design (which plays into a))
c) Poppet design (which plays into a))
3) Air Density (which plays into 2))
4) Component Quality
and lastly 5) 1st stage contribution
1) Cracking Effort
Basically, how hard do you have to suck to open the valve? This is the single biggest factor, and fortunately, the easiest to fix.
My fiancee trained on my regs. But when she went to Hawaii for her OW dives, she air-shared on her Instructor's rental octo. It cracked so stiffly during the air-share ascent, she almost bolted for the surface in fear that she wasn't getting enough air. Fortunately, she kept it together, but surfaced with a new appreciation for well-tuned gear.
2) Dynamic Flow
a) Most modern designs are deliberately shaped to make use of the physical principle known as the Venturi effect. Basically (an oversimplification) when high pressure air goes thru a narrow opening (like your valve orifice) it tends to speed up, and the pressure on the downstream side drops. Inside a scuba regulator in mid-breath, the drop in pressure is almost enough to equal the suck you make to open the valve, and therefore keeps the valve open without you exerting any added effort. That's "easy to breathe"! If excessive, however, the reg will cross over into freeflow, as may happen when a reg hits the water without your mouth on it to offer any back pressure. Poor design or tuning can mean that there's no improvement in effort of breathing
during the breathing cycle, in addition to what it takes to open the valve.
b) Case design - in the old days, a good breather was almost accidental. The Scubapro 156/Balanced Adjustable was hand brazed together with separate brass pieces before chroming. The case was ?accidentally? well shaped, but some regs delivered great flow, others didn't. But it didn't take long for manufacturers to recognize this contribution, and (for example) the Scubapro G250 consistently delivered great dynamic flow, with extremely low resistance mid-breath.
c) Poppet design - The Scubapro Pilot is the standard against which all other valve designs are measured. Extremely complex, it was also center-balanced. Without going into what that means, what that shape also has is an extremely short air path between the valve opening and the mouthpiece. Most all "standard" barrel design regs today instead have end-balancing, which necessitates that the air path leaves the valve, and travels down over the poppet and spring before making a turn out toward the mouthpiece. This affects dynamic flow, and must be accounted for when Venturi effects are added by design.
3) Air Density
When you dive deep the air literally gets thicker. At 132 feet, it is five times as dense as at the surface. That makes only a small difference in modern designs, but deeper than that, the sheer mass of air molecules has an increasing effect on Venturi pressure drops inside the case. This must be compensated for by design adjustments or other changes that can be made on the fly. It's complicated, but for the Recreational Diver is of little significance.
4) Component Quality
By this, I basically mean friction. Components that are poorly finished slide less well over other poorly finished components. This translates into greater effort at making things move, and without regular lubrication, can translate into "stiff" breathing.
5) First Stage Contribution
In the old days, many first stages were unbalanced. That meant that, if they were a piston design, the pressure they supplied to the second stage dropped as the tank emptied. At the same time, second stages were also unbalanced, which meant that the valve opened easiest when the pressure against it was the highest. With an empty tank, the result was predictable: decreased tank pressure equaled decreased "intermediate pressure" supplied to the second stage by the first, which then meant increasing effort (suck) to open the valve.
Nowadays, at least one of the two components is "balanced". Without going into detail, balancing means that (in a 1st stage) the pressure supplied stays more stable as the tank empties, or (in a 2nd stage) the negative pressure (suck) needed to open the valve is more stable over a wide range of supplied intermediate pressures.
What's the bottom line? Most modern designs of even the most inexpensive brands (due to competition in the marketplace) perform near identically IF they are tuned to the same cracking effort, WITHIN recreational depths. That means, if you feel a difference between your reg and your buddy's, it's most likely due to a difference in cracking effort, which can be solved with a quick retune.
The consequence of this improved design and manufacture, even with inexpensive gear, is that preventive maintenance on scuba equipment is often REALLY neglected, to the point that you don't see a difference until you're reaching that shoulder of the curve just before you "fall off the (performance) cliff".
Use
@couv's
Regulator Inspection and Checklist (Rev-8)
Read Pete Wolfinger's "Regulator Savvy"
Scuba Tools (a bit of a yawn, except for equipment geeks).
Or at least, find a reg technician that you can trust. And that's a subject for an entirely different thread. Because "that guy" at the beach who can fix your freeflow, or improve your tuning, may not have any clue about the secondary (and potentially dangerous) effects of an isolated cracking effort change.
