For me it was an altitude/temp combination under load. Mostly the truck was fine, but dragging 3000lbs of trailer 2000ft up (I know, nothing compared to out west) at 94F ambient and 80+% humidity the truck was a dog trying to get up the grades of backwoods KY. I ended up with a SC, but there was an interim step where I was extremely happy for the cost. It was a combination of the aFe MomentumGT CAI and URD's MAF Calibrator.
Adding a CAI doesn't do much if you're not adjusting the computer for the additional air flow. This is especially important if you've changed the diameter of the tube near the MAF sensor, between the air filter and the throttle body. Adding a Sprint Booster or the like will change the throttle mapping, kind of like going to sport mode instead of eco-mode in some other vehicles. URD's MAF Calibrator will correct the readings of the MAF Sensor to be appropriate to the diameter of the inlet pipe and amount of air flowing through it. There was a noticeable (though not large) performance increase over stock in this configuration.
Also what octane fuel are you using? The engine will automatically adjust timing on a variety of variables. The smaller partial pressure (thinner air) at altitude means advanced timing because it takes longer to burn. Because of its tendency to pre-ignite, low octane requires retarded timing. The idea of 85 octane at altitudes came with vehicles pre-1984 when timing had to be manually adjusted. At that point the thinner air meant you could run a lower octane fuel which costs less and balanced with the timing advance required for higher altitude operations, hence less tuning and adjustments required. It was simpler for lower octane fuel to be sold in high altitude areas than to adjust the timing of every vehicle when they happened to drive through those areas. For those who live there, a timing adjustment was a better long term solution. On modern vehicles the computer will automatically adjust timing, so this is not as big of a deal. However, if you want more efficiency (which can translate into more power), try running a higher octane. Just know that it can take a couple full tanks for the computer to adjust to the better efficiency (or you can reset it by disconnecting the battery for about 2 hours).
Like anything it's likely a combination of multiple factors, high altitude, ambient temps (summer vs winter), lower octane fuel?, larger diameter tires, wider tread (more friction), lift (increased air resistance), and even more!
FWIW I also didn't care for the rough ride on the KO2 E load tires and dumped them in almost new condition less than a year after I bought them. Now running the Cooper Discoverer AT3's and they're much better.
