Ever tried picking ammo out from ammo box with generic combat gear and after moving and running around for a while or even in odd position (like prone) ? And especially in low temperatures (below -15 degrees celsius) or high temperatures (20+ degrees celsius) ?
Been there, done that. In fact, I do monthly military rifle competitions with my local gun club. We've done shoots in temperatures ranging from -25C to +40C (that royally sucked in a German wool uniform), with positions including standing off-hand, kneeling, sitting, and prone.
Generally I do strictly Russian and German rifles for these events, mainly because I have a variety of Russian and German rifles, and the uniforms and fieldgear to go with them (2nd Guards Army, 287th Rifle Division for Russian and 2nd SS 'Das Reich' Division, 'Deutschland' Regiment for German). Not only that, but I also do reenacting as both Russian and German with a reenactment society out of the northwest region of North America.
So, yes, actually, I do know what it's like to have to load under duress and in a wide variety of situations and I still say that the reload times, especially for the Germans which have one of the EASIEST stripper clips to load from, are horribly slow.
However, I do agree that the reload times should vary based on your position, if you're under fire, if artillery is going off around you, etc.
Stamina not so much. I can run full tilt for a few hundred metres in full battle kit and still reload in the same position just as fast as I could if I'd been sitting around all day, but definitely for most everything else.