Zaps
The fact "people can slow down the game if they want" is not a good reason to allow super fast speeds. Many players (myself included) will feel disadvantaged if they don't play at the fastest speed, especially on a competitive server where there's a race to achieve max level. Personally, I prefer to play at maybe 3/5 max speed, but I often feel like I'm crippling myself. Consider what would happen if there was ultra-ultra fast speed, where every combat round completed instantly. Some players would feel obligated to play at this speed to level/collect loot faster, but the game would essentially be reduced to a text based game where the only way to know what happened is to study the combat log.