Some skills have a Casting Delay. A Casting Delay is a period of time after a player has cast a spell during which the player cannot re-cast that spell. For example, after casting Blizzard you must wait a few seconds before you're able to cast another one. The delay differs between skills. Not all skills have a Casting Delay. There is nothing you can do to reduce or eliminate the Casting Delay. If the Skill Action Button becomes red after casting a spell and you have enough Mana to cast it, this means the Casting Delay is active. This is done to prevent slow down of graphical performance and to avoid abusive use of the skills.

Tip: While you're waiting through the Casting Delay you can switch to another spell and use it while you're waiting for the first skill to be available to cast again. Example: Cast a Fire Wall (Casting Delay), then quickly switch to Static Field (No Casting Delay), then switch back to Fire Wall again. Once you get your timing right you can constantly cast spells even with a Casting Delay on one or both of them. Casting Delay spells are connected, such as the Druid's Armageddon and Hurricane, so you may not be able to combine two of the same Casting Delay spell.

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Poison Javelin - 0.6 seconds
Plague Javelin - 4 seconds
Valkyrie - 6 seconds
Immolation Arrow - 1 second

Assassin
Dragon Flight - 1 seconds
Shadow Warrior - 6 seconds
Shadow Master - 6 seconds
Shock Web - 0.6 seconds
Blade Sentinel - 2 seconds

Druid
Firestorm - 0.6 seconds
Molten Boulder - 2 seconds
Fissure - 2 seconds
Volcano - 4 seconds
Armageddon - 6 seconds
Hurricane - 6 seconds
Werewolf - 1 second
Werebear - 1 second

Paladin
Fist of the Heavens - 1 second

Sorceress
Blizzard - 1.8 seconds
Frozen Orb - 1 second
Fire Wall - 1.4 seconds
Meteor - 1.2 seconds
Hydra - 1.6 seconds