Originally Posted by tetracycloide

Haste for Casters:

Haste has a cap on effectiveness as a result of the global cooldown being 1.5secs. Sufficent haste would push many spells to less than 1.5 secs of cast time but they would still only be castable every 1.5 secs due to the gobal cooldown requiring a minimum of 1.5 secs between casts.

However, haste will still reduce the actual time spent casting which can be advantageous when dealing with spell push back or trying to avoid interupts. Haste will also reduce the amount of time before the first cast which is also situationally advantageous when trying to move and cast at the same time or other situations that encourage narrow time windows for casting.

Channeled spells are effected by haste and will see a reduction in their time spent channeling the spell but no change in overall damage resulting in a DPS boost.

Haste scales at the same rate for channeled spells and spells with a standard cast time.

Instant cast spells will gain nothing from haste.