It doesn’t. We’ve seen countless of times of Newgate and teach activating the gura aura at will.
Akainu is the same he makes his hands into magma fists just as Newgate makes his hands into gura fists.
Nobody in the series has to charge up their attacks unless it’s like a Hakai/high-end
Again they used it on and off.
I have never seen them spamming it casually.
Akainu take some time to charge his punch,
Hence why Ace, Crocodile, jinbe, Marco , Shanks intercepted him before he reach his target