throw 10 fire spells. If 4 of them trigger firestarter, that'd 40% proc rate.
if we jump the gun and cast ahead, in a fair manner, it'll be 2 of them trigger firestarter... and that's 20% proc rate out of 10 casts in general.

weaving in a skill would take 0.5 s, 1 spell takes 2.5, so if we weave 5 times, we'll lose a chance to cast one spell while maintaining the 40% proc rate.

so which one is more efficient, under the same given time frame? 6 casts with 20% proc or 5 casts with 40% proc? But please keep in mind, the more spellspeed would upset this equation (because it doesn't affect the 0.5 s delay.)