So I played the demo first on wait mode (by default, like anyone else), and then after I played on Active mode, so I would want to give my impression after having played both.
Wait mode : By default, you get that + normal battle speed when you start the demo. My time with the combat system was fairly easy. I could take whatever the time to think about my strategy, although they were simplistic choices, I had all the time to focus on what to attack/cast next. Overall, it was ridiculously slow and easy. It let a bitter taste in my mouth when I finished the demo (like, was I the target audience or what?). The battle UI was different but interesting, although I liked to be able to use shortcuts to cast spells and abilities quickly (that you can define in the main menu beforehand)
Active mode : Now, what I did is that I went straight away in the option menu from the main menu and I selected : Battle speed - 5, Active Mode and Classic UI and I started the demo. Now, the experience changed a lot for me (and in a good way), not only because the battle UI was far more enjoyable to browse, but because the monsters didn't wait for their turn to attack me, like a real ATB system. I can say that at the beginning, I struggled a lot in front of those tonberries and mandragora, just because you need to think quickly before getting attacked from all corner by Knife Tonberry's OP attack. I sensed overall that this batte mode + fast battle speed is the way to go if you want a fairly challenging experience in the full game.
On the other hand, you need to think about why it was set on wait mode by default : You guessed it, WoFF is targeting a younger audience who wants to be introduced to Final Fantasy, so actually a casual market first and foremost. So if I would give anyone here who are not feeling it right now or would want to try this demo for the first time, give yourself a treat, and start the game in Active mode + fastest battle speed + UI Classic mode.