Projectile speed. How to decide which one is better?

What is the deciding factor to choose either more speed or reduced speed?
Is it only the ailment inflictment chance?

It's based on the mechanics of the skill.

Spectral Throw, Ball Lightning, and Molten Strike work better with less, because they will be able to get more hits on a single target that way.

Spark, Freezing Pulse, Ethereal Knives, and Frost Blades work better with more, because they will be able to have a greater reach.

Frostbolt works better with more, because it just feels bad if the projectiles are moving so slowly.

If you use bow attacks, there is also "Increases and Reductions to Projectile Speed also apply to Damage with Bows" mastery.

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Echothesis wrote:

If you use bow attacks, there is also "Increases and Reductions to Projectile Speed also apply to Damage with Bows" mastery.

Note that it says "Increases", not "More".

"More projectile speed" does nothing for this mastery.

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jeerinho wrote:

because they will be able to have a greater reach.

why is that?
do they have a limited active time?

I assume lightning arrow falls in the faster is better category?

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LoTharios wrote:

why is that?
do they have a limited active time?

I assume lightning arrow falls in the faster is better category?

Yes. They dissipate after a certain time. Having faster speed allows them to travel further before that happens.

Projectile speed has very little effect on LA. It is not as mandatory as those other skills, other than to have synergy with the mastery "Increases and Reductions to Projectile Speed also apply to Damage with Bows" mastery.", as mentioned by Echothesis.