I never knew this, but slow spells can require multiple casting confirmation turns before the caster actually executes the spell.
I wanted to verify the spell ranges (fixed quite a few I had incorrectly written years back because I forgot to account for the S-grade skill that was adding 1 to the spell range) so I put a bunch of runes on some max level mages with no skills and just had them cast some powerful, slow spells like Earthquake and Final Flame for no particular reason in order to move on to the next mage's turn for more spell range checking. To my surprise, I had to use 1 turn each to "reconfirm" whether to continue chanting or cancel the spells, and it took a 2nd confirmation to actually execute them. I know that the MAG stat affects chanting speed, so it may be possible a really poor MAG stat (like Pablo when you first recruit him) might require 2 or even more re-confirmations to continue chanting before you can actually get to the spell execution confirmation.
this is similar to Suikoden III, but that game plays differently, since everybody gets an action each round and you always have to input new commands.