When it comes to casting time (and casting in general) Initiative is key. Try to maximise the Initiative of your caster, by using items that give bonuses to it, and keep the encumbrance down on light or you get a penalty.

Knowing when and where to cast is also important. If the enemy is close together and you are certain, that you can finish casting Fireball or Blizzard before he is able to spread out too far, you should do it. It takes some practice, but it is very effective. The best time to do it is, if the AI is fielding lots of ranged units, because it usually won't advance its melee units unless you do it first or the ranged units get disabled (killed off). This is especially apparent during city combat.

The mana cost is only an issue early game. There are a couple of ways to reduce it. Path of the Mage reduces the cost by 25%, the Affinity-trait by another 10% and there are several robes that reduce it even more. If you have a champion with access to both Life and Water magic, you can get Mantle of the Ocean, which reduces the cost by 40%! Add all of those together and you can reduce the cost to almost 0. Even if you don't get all of those, depending on the amount of Shards and Conclaves you have, you can get a Mana-income high enough to cast those spells several times per turn.

While the damage of your spells is relatively low at first, as soon as you get some appropriate Shards and damage-increasing traits, you can lay waste to entire army with one casting of Fireball or Blizzard. The casting time is more than justified in that case. You wouldn't want this to happen to you without a chance to prevent it or at least minimise the damage.

Regarding disrupting spells: the only way to do this by force is to kill the caster. The other way to do it is to use one of the three counterspells (Counterspell (Path of the Mage), Feedback (Fire III) and Spell Leech (Water III)), but they require you to beat the enemy's Spell Resistance to be successful.