I just tested it out. It is difficult to tell the difference with just one micon line, but if you stick a bunch of them, it is easy to feel the lag. The one place where I could really notice the difference is in looking at the icon. When you hit a button, it makes a little circular animation on the button every time you press that button, and then the skill darkens for GCD. I noticed I could see more of that circular button press animation on the one micon at the end, meaning that because of the slight delay in the GCD being executed, a split second less of the button press animation was visible.

Hopefully that wasn't too confusing. Bottom line is Mikoko is correct about the micon taking time just as any other macro line.