Cast & attack speed with priest

[Aionyx]

Blood or Holy Tree skills have nothing to do with this. It is the animation times which are affected.
For skills with greater casting time (i.e., greater than 0 sec) it is better to have higher casting speed. For example, Light Heal takes a 1.5 sec casting time. If you get, say 4 Immaculate Kyanite Gems (thus increasing the Casting Speed by 60%), it would come down to about 0.6 sec.

Attack speed refers to the delay between successive attacks (or attack animation :P). It affects the "duration" of the skills - usually the default skills. Take, for example, the Blood Beam. It has a duration of 1 second. Increasing the attack speed will shorten the delay between successive attacks. In this case, 4 Immaculate Pink Diamond Gems (increases attack by 60%) will lower it to 0.4 seconds. You can get in more attacks this way.
This is the reason rogues get in so many attacks and why most priests flounder since they don't invest in attack speed and, thus, fail to survive longer and deal more damage over time.

+1 that summed it up preety much eventho i'm not a mage or priest,i have friends who are and they invest in atk speed,its essential