I'm not sure what you're basing this on. In my book, anyone who folds out the feet is doing it wrong and I can't imagine I will ever be convinced otherwise. Inclined keyboards are for hunt-and-peck typists.
This stuff is all mechanics/common sense that is relatively simple to empirically verify.
1. You're not sitting up straight
This is basic to increasing productivity and competence in many activities; it's even the first lesson in speed-reading courses. The whole point is to increase oxygen to your body/brain by maximizing your lung capacity and diaphragm movement.
2. Your board is at a level that doesn't allow your elbows to form a 90deg angle
The height of your kitchen counter isn't arbitrary either. Having a work surface that allows your arms to be at a 90deg maximizes movement and leverage; and in turn comfort.
3. You're resting your palms on the desk or a wrist rest whilst typing
Unless you're Arsenio Hall, the function row is likely inaccessable and the corners of the number row are probably a bit of a reach if not at a painful angle when you're resting your palms with fingers on the home row. Some ergonomic boards are designed to rest your palms, but standard layout boards are more like pianos; keeping your palms glued to one place is going to limit your access to distant keys and will slow you down (lift/rest/repeat) if not make reaching distant keys uncomfortable. My hands are of above average size I'd gather, and stretched out measure 19cm palm to the tip of my middle finger and 24cm thumb tip to pinky tip and I still can't reach all the keys on an HHKB without lifting my palms in the air.
4. You didn't fold the keyboard feet out
The angle of the keyboard and keycaps is not for visibility of the letters; it's two-fold rather: one, to minimize the distance your hands need to move so your fingers can access different rows, and two, to provide better leverage for the angle your fingers attack the keys at. Even if you are a palm-rester, you're going to have better reach on an angled board than you will on a flat laptop (granted the same key spacing), it only takes a ruler to verify this.