There are two variables that determine how quickly a character will perform an action; one is the character's speed, the other is the energy cost associated with that action. Whereas speed reflects the overall quickness in thought and action of the character, energy cost reflects the player's particular quickness with that one particular action.
ADOM is a turn-based game, but does simulate the flow of time and different speeds; the system it uses is described in detail in the game manual. In short: The game works in so-called segments. During this segment the creature in question accumulates energy points equal to the creature's speed score. Once a creature has gained 1000 energy points (or more), it is allowed to perform an action. Actions will lower the character's energy points by a certain value; this is the energy cost. This energy cost will drop the character's energy points below 1000; they will be allowed to act again once enough segments have passed to get them back, and so on for infinity.
The base energy cost is 1000. Most actions that are not otherwise stated to use more or less than that will cost 1000 energy points. (An exception to this are throwing attacks using improvised throwing weapons, which consume 1200 energy points.) With a base speed of 100 this means that the player can perform such an action precisely once every 10 segments. This base energy cost, however, can be modified by specific actions. For example, performing an attack with melee weapon only costs 1000 energy points if the character is completely untrained with the weapon; with a skill of 1 it costs merely 980 energy points. (An elaborate table is located in the manual.) Many class powers significantly reduce energy points for certain actions the character is particularly talented in; for instance two Archer class powers significantly reduce the energy cost of missile attacks (these rank among the best class powers for any character in the game). The magical item seven league boots halves energy cost for walking (in 1.1.1 and prior, in 1.2.0+ the reduction is only 25%), making the character incredibly fleet of foot.
Energy cost reductions are often expressed in absolutes in the manual ("takes up but 800 energy points"). In truth, these function as percentage reductions; these statements in the manual always assume that the regular cost is 1000. The level 12 Archer class power does not set missile attacks to cost 800 energy points, but rather provides a 20% reduction (800/1000). So if the Archer has level 1 in the relevant weapon skill, the energy cost of a missile attack becomes 784. Energy cost reduction stacks in this manner; as percentages. This math still applies to some higher than normal energy cost special actions, such as the Barbarian mighty blow; using a mighty blow only costs 2500 energy points with an untrained melee weapon and without other energy cost-reducing factors applicable. With a weapon skill of 1, the 2% (980/1000 is 98%) reduction in energy cost is applied, for a mighty blow energy cost of 2450.
Players familiar with the concept of "dps" - damage per second - will quickly realize that reducing the energy cost of their melee attacks improves the character's efficiency in combat greatly. One advantage that having low energy cost has compared to having high speed is that energy cost reduction will not entail the XP penalties associated with high speed.
It is speculated what there is a limit to the amount of energy cost reduction you can pile up, in particular the number 336, or roughly 1/3 of the norm, was reported. Though the only way to reach this threshold is to get the Archer's level 25 class power (40% reduction) as well as Quick Shot and Lightning Shot and very high skill in the chosen missile or thrown weapon. On observations with lv50 + both talents at some point improving missile weapon skills stops reducing attack energy cost. This is pretty much a theoretical question as none of the other classes come close to this, except maybe monks for movement: Class powers + blessed seven league boots + Long Stride adding up to 369 instead of 1000 for movement. The only issue the cap may cause is to ask whether Archers should pick Lightning Shot, yet considering the cap can only ever be reached with level 50 archers, it's mostly irrelevant.