It wasn't impossible to cast spell instantaneously. If one had high level of mastery over a spell, they could do it.
For example, on earth, when someone try to ride a bicycle for the first time, they would find it very hard to maintain the balance, let alone ride it.
However, after getting familiar with it, they could do it without any effort. Some wouldn't even need to hold the handlebars to keep it stable as they already have a significant level of mastery over it, and could do multitasking like talking on mobile phone etc.
The same rule apply on the magic spell too. The more you use it, the better you become.
Max for example, could now cast his fireball spell at a moment's notice. Because he had used it some many times, his body and mind had become extremely familiar with it.
