Final attack

It delays your next attack because it procs to do the final attack. Damage wise its best to skip it so you can continue keeping your DPM up instead of constantly being held back by the final attack procs.

 

WE. DO NOT. TALK ABOUT. FINAL ATTACK.

On a serious note, AFAIK FA makes you lose a lot of damage when you use Slash Blast. Simply put, FA damage is divided between all the mobs you hit, so you deal less damage as compared to using Slash Blast again, and sometimes you fail to knockback some of the mobs.