SE and I obviously disagree on the definition of the word "instant"

[Zalestra]

Macros will wait for your castbar/GCD to be completely at 100% before they can be fired and its checked on client side, so lag is a factor.
Without a macro, you can start casting the next spell /action a bit before it is at 100%.

Also, lets say you macro Foresight.
Without a macro, you can press foresight while you are on GCD, and foresight will be activated right when your GCD finishes. (Thats reffered to as skill queueing)
A macroed forsight can only be pressed when you are not on GCD.
Same goes for any oGCD ability.

Thank you, that explains some things for me. Especially for my raise macro. Sigh, I will miss all the macros I will be killing tonight (´Д｀)