Speed indicates the attack speed of your character. Each second, you gain a number of action points (APs) equal to your Speed rating which can then be spent on performing combat techniques. This statistic is rounded down to the nearest whole number, and is calculated as follows:
Your Speed has a maximum cap (usually 25-35, depending on encumbrance). Each full point of Speed beyond the Speed cap will be converted into 1 point in each of Attack, Defence, Ascendancy and Protection, applied at the same time as your primary stats.
Your maximum action points are 800, plus 20 for each point of Speed. Your true Speed stat (before the cap) is used when calculating this maximum.