30% to cast speed is huge but does it simply multiply by 1.30 or does it add +30% to an existing value?

I think it's the second one but I have no clue what is the base cast speed on an outlander... Looking for casts per second on a char with zero +cast spd.

I found myself throwing glaives at an estimated 2 glaives a second or so, maybe 2.2/sec but that's too rough for me.
with that estimation, a 30% cast improvement will take my cast speed from 2/sec to 2.6/sec correct?

I would say +30% of the base cast speed but i cant be 100% sure.

IT is off the base cast speed, you cast 30% faster. it would be 1.30, 2.0 being 200% or twice as fast.