So, obviously, having an infinite amount of whatever element you gained off of the logia devil fruit you ate would be powerful. Being able to freeze anything with your ice powers or crush your opponents with sheer force from magma instantly would grant you the win against any normal citizen. However, I don't quite know how fast they could produce their element. I suspect that it is based on mastery, because of how Luffy couldn't control his body when he first gained his powers, but if so, how does mastery effect the creation? Like, does that mean if you had 10 years of devil fruit training you would be able to produce it instantly?
First of all the Gum Gum Fruit that Luffy ate is not a Logia, its a Paramecia type. Logia fruit cannot make infinite amount of element and it is directly proportional to the eaters energy level.
A Logia fruit eater can produce element instantly even on their first try.
We can see that when Sabo eat the Flare-Flare fruit in Dressrosa arc.