They should just delete AST

[Mortex]

Entirely theoretical, and only applies when protracted over a long period of time.
In a 30s window you’ll have about 14 GCD’s max, or 12 if you haven’t stacked any Speed materia.
That’s not enough time to reap the benefits of a 5% increased speed buff. Your 13th/15th GCD at the end of that 30s window will come a second or two quicker, and not even a whole GCD quicker. That’s it. You’d need to be able to fit 20 GCD’s into that window to gain one whole extra GCD, and only Ninja could do that, but that would cause clipping and therefore cancel out all benefit of it.

If it was a permanent buff throughout a fight it would make a difference.

That’s the math for every single aoe arrow. Other people even confirmed it. You either don’t want to believe math for that. I can even give you even the thread that has the math.