I haven't had any trouble doing content so far, but I find that the margin of error is too small considering the lack of responsiveness. For the record, my computer is amazing, so hardware does not factor into this.

Let's take an easy example: trying to stun a caster mob.
Cast takes let's say, 3 seconds.
So, mob starts casting. It probably takes about .5s for me to notice it's the "Cure" spell, because I have to read it since all spell casts look generally the same minus the text.
Takes another .5s+ for the key to be pressed, the game to recognize it's been pressed, and even begin to process the idea of this ability.
Takes another FULL SECOND, for my character to go through a ridiculously long animation for the ability.
Takes another .25s or so for the game to register that it hit.

I would expect to have at least 1.5s to react to a 3s cast bar, not less than a second.