Combat is based on rounds. Rounds are periods that last for six seconds. The Speed Factor of a weapon determines when within those six seconds a creature wielding that weapon hits. The lower the number, the earlier the hit. A Speed Factor of 0 means that the creature hits at the start of the round. A Speed Factor of 10 means the creature hits at the end of the round.
Attack animations take 1 second to display. If a character attacks with a two handed sword (Speed Factor 10), the game starts the attack animation 5 seconds into the round so that it finishes (and the hit is delivered) just at the end of the round.
A level 13 fighter with 2 APRs wielding the same weapon, but with two attacks this is now split into two three second periods. The weapon will still strike at the very end of the period, but now this will be the end of the 3rd and 6th seconds (again, with the animation beginning slightly earlier to allow the blow to land at the right moment).
So what happens to having more APRs? There are weapons like Belm that give +1 APR, an Improved Haste doubles APR, and there are Greater Whirlwind High-level class abilities (HLA) that set APR to 10, the maximum amount the game allows. Well, you’ll never exceed 5 APRs no matter which weapon combination you choose or how you wield them, as for IH or GW, the game actually splits one round into two (3 seconds each) and doubles the attack animation speed (0.5 second each) under their effects. Imagine a fighter wielding the same weapon activates GW, for the first 3 seconds, s/he has 5 attacks to complete, the animations will take up to a total of 2.5 (0.5x5) seconds, assuming no pauses between them, so the speed factor has only 0.5 second period to factor in, which is almost negligible. Speaking of GW, the noticeable delay after the HLA activation will surely make its effect go off before the 10th attack finishes, a slight oversight...
To sum up, Speed Factor is noticeable for characters with low APRs, but matters less for characters with high APRs.
Things that affect Speed Factor:
- Weapon type
- Weapon enhancement level, usually is equal to the SF bonus
- Weapon proficiency, High Master -1, Grand Master -3, Two-Handed Weapon Style -2 or -4
- Class kits, Kensai reduces SF by 1 every four levels
The game also rolls a random initiative factor each round so that even with a worse speed factor, attack can come first.