projectile speed?

whats the purpose of projectile speed? does it increase dmg?

It… increases the speed of your projectile(s).

I’m not sure why you would, but if you were building around Magma Orb, you’d probably want some. It’s usually more of a QoL fix than anything else. It doesn’t improve your damage, no.

If you’re using, say, Tornado Shot, then you’re probably fine with the default speed and any incidental sources of Projectile Speed you happen to get along the way. I remember wanting some when I played a Spark build back in the day, just ‘cause the little buggers are a bit slow and they wander around randomly. Most projectile builds probably don’t care for it - but some do.

All projectiles have a duration before they "die", whether the duration is explicitly mentioned in the tooltip or not (it's usually not). Projectile speed allows projectiles to travel further before they expire. This can be quite useful with certain skills/builds, such as Spark since it was already mentioned.

It also makes projectiles arrive at their target sooner. In a lot of cases this isn't super important, but for some skills it can be quite good. Especially those that can burst fire a lot of projectiles and you have various "on hit" effects that will apply with each hit. Barrage is a pretty good example of that.

There is a Bow Mastery that you can allocate on the bow passive cluster so investing in projectile speed has synergy.

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Increases and Reductions to Projectile Speed also apply to Damage with Bows

You can also check here what it does to a certain niche build:
https://www.youtube.com/watch?v=-ErIQD2WBvY

I think most of the time stacking projectile speed by accident won't harm the build, except Molten Strike, which needs slower projectile speed to optimize the damage. EDIT: Almost forgot that Ball Lightning and Spectral Throw also needs slower speed.