Clean is clean and dirty is dirty. Standing timber is generally considered clean, though some trees in some conditions will catch windblown sand and dirt in their bark. Dirty wood is usally wood that's been falled, and has been drug, skidded, or just been laying on the ground and is literally dirty. Dirt is your chain's #1 enemy. It's all about cutter design, but the long and short of is that some chain designs will hold a greater relative amount of their cutting ability in dirty or clean wood than others.
Stihl's RM cutter is better in dirty wood, and Their RS lines are better in clean timber.
You say you don't sharpen. Why not? That's basic saw maintenance man. That's like putting gas in your saw. You gotta do it. A sharper chain will make up for saw performance any day. A dull chain will drag your saw down, cause wear and tear, and will work you harder. Even if you take it to the dealer to have them do it, you should still be able to touch it up yourself. You say "when it needs it." That's a couple of times throughout a day of cutting. Learning to file by hand is simple as can be, and is a "must-do."
