Ah. You forget the game system.
EVERYTHING in D&D including walking down the hall, chatting in the tavern, combat and traveling takes into account WIDE (and sometimes Wild) AVERAGES.
An average horse from our world walks faster than almost any walking creature in D&D...including a horse (in fact someone here posted the average speeds of many animals including an Olympic long distance runner...it was quite amusing). Anyone in good shape who's walked a 20 mile Walk-a-thon in 6 hours knows this. But the system takes into account that occassional steep incline...the fallen tree you have to go around. That narrow stream you have to search to find a safe way across...you hopping off the horse to pop something out of its hoof...or stopping to take a hasty dump.
Same thing is true for combat actions. As a trained fighter, I can attack you with my sword many times, and you can attack me just as many in the space of time alloted by D&D to attack once (this fact was the main reason why combat times dropped so much since AD&D (the 1 minute rounds)...too many D&D players are also martial artists or SCA fighters and they just knew better...hehe). It's not realistic, sure but at least its much better than the old system. Fighters like me have to remember that D&D fighting takes into account the "turbulence" of MORTAL combat...the faints, parries, the chaos, the pain, and death! Also, real life fighting doesn't take into account that the guy I'm fighting happens to be made of fire...or that we're waste deep in trapping vines, or that, as I'm looking for an opening, the guy too my left is chanting and gestruing and talking about how badly this lightning bolt is going to hurt, or that any second, this big animal thing might open its mouth and spit freakin ACID on me...etc, etc.
NO PROFESSIONAL (or otherwise) beer drinker can get his LIPS on the glass with me swinging at him, let alone drink from it. The d20 system offers that happy medium by allowing it to happen (as preposterous as that is) by averaging the time it might take to drink it while sitting relaxed on a stool with trying to hastily grab a sip while an Ogre is trying to smash your head in with a tree stump. It's all about averages. Considiering this, it's really not all that unreasonable to drink a potion in a standard action (I for one have a hard time drinking a capful of medicine and CANNOT do it in a standard action). Makes sense if you remember the context.
If you don't believe it, get a friend, preferably someone who doesn't like you and have them slap box with you while you try to drink a glass of water. Then come back and tell us how you did!
