Attack speed describes how fast a unit can attack. It is measured as the weapon's period or speed, a number equal to how many game seconds the unit must wait between attacks. Attack speeds are also affected by a random delay, the effects of any multiple attacks the unit has, and other effects (e.g. Stim Pack). The minimum attack speed multiplier is 0.25, and the maximum is 128. A weapon's cooldown, which is accessible in the editor from the function "Weapon cooldown" (of type Real), is 16 times the period.
There is a random delay added to (or subtracted from) the attack speed of a unit each time it attacks. The random delay is usually randomly selected between -0.0625 (faster) and 0.125 seconds (slower), but the following units have different bounds for the random delay:
For example, the D-8 Charge's period is 1.8, which is shown as its speed in-game, but with the delay it takes between 1.9 and 2.3 seconds between attacks. The random delay usually has minimal in-game consequences, but it does mean that when multiple of the same unit type attack a target together for an extended period of time, their attacks can drift apart from each other instead of being at exactly the same time. It also means that it's possible for one unit to lose to another of the same type even though it attacked first, though it's more likely that the first to attack will win.
The fastest possible attack speed is 0.0625, or 1/16. The slowest attack speed is 32768. In the early beta, attack speeds were shown in-game as names instead of numbers. The cutoffs for which word described the speed are below. When an attack period was at the cutoff for two labels, the slower one was chosen. e.g. a unit with an attack period of exactly 0.75 would have been listed as "Normal", not "Fast".