Back in my early days of gaming, when AD&D 1e was all of the rage, we tried to stick as by the book as we could (although we never did understand weapon speed.)
Failed a save against a fireball? Time for everything you own to save too.
Casting a spell? What's the casting time? Bamn! You were hit! Lose your spell. Try again later.
Gradually we found ourselves dropping certain rules in favor of expediting game play.
One of the first rules we dropped was counting segments for spells and interrupting casters. It changed the dynamics but certainly made enemy casters more dangerous, as they always seems to have their full complement of memorized spells when the PCs encountered them. Go figure.
So, do you allow spell casters to be interrupted while casting? Do spells take a certain number of segments to cast in your campaigns?