ToME WikiEverything on the map, player and monster, gets a certain amount of energy every turn (these are the game turns). You normally get 10 energy a turn, but that varies with your speed. The higher your speed, the more energy you get. It's mostly linear, up until around speed 30, where it starts to taper off rapidly (+40 speed does not give +40 to energy; I don't think +40 energy is even possible). Likewise down below, -10 speed does not mean 0 energy a turn.
You get to act when you accumulate 100 energy. So at speed 0, 10 game turns equal one player turn.
Most acts will spend that entire 100 energy. Special speed effects like extra shots, the speed effects on spells from things like Mage Staves, reduce the amount of energy taken for specific acts. For example if you have an extra shot with your bow, firing it will only cost you 50 energy, so you effectively shoot twice as fast, because you're getting turns twice as fast.
Duration of spells is measured in player speed + 0 player turns.