Speed Factor determines at which point in the (6 / APR) second(s) period in one round the creature will strike, 0 means instantaneously, 10 means at the end of the period. To elaborate, take the basic Two-handed Sword as an example, which has a speed factor of 10. When you issue an attack command to a character with 1 APR who's wielding the weapon, the point should be set to the end of the 6th (6 / 1) second in the round, however as the attack animation itself takes 1 second, in order for the attack to complete within the round, the game will allow the character to start attacking at roughly the end of the 5th second (or the beginning of the 6th second) and finish the attack animation just at the end of the round.