The PHB states that a character can provide first aid to stabilize a dying character. The Heal DC is 15 and when the character manages, the character immediately loses HP and is stabilized. This requires a standard action. (Assuming you receive a turn and a standard action in a round and a round takes 6 seconds, this action takes about 3 seconds.)
But on page 138 under "The Combat Round" it says
Anything a person can do within six seconds can be your character doing in one turn.
My problem is the discrepancy here. They can not reasonably stabilize someone and prevent them from losing blood, etc. in 3 seconds. Without magical help it is impossible. So how is it that a person can perform a heal-check and stabilize a dying person within 3 seconds, if that is unreasonable?
In the light of these two contradictory rules, how long does it take to use First Aid to save a dying character?