Which magic schools should I choose for the late game? Or should I go for a specific artifact?

I'm talking about mass slow, cleanse, and haste spells. With the Mage's Sign, you can only haste two units, and it will cost you 1.5 turns... 0.5 for the Mage's Sign + 1.0 for the spell itself... But if you have a Mind Lord, it only costs 0.5 turns, and it affects everyone... That's why I stopped using the Spell of Acceleration (SA)...