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

I'm talking about mass slowing, cleansing, and speeding up. With the Mage's Sign, you can only speed up 2 units, and it will cost you 1.5 turns... 0.5 for the Mage's Sign + 1.0 for the spell itself... And if you have a Mind Lord, it only costs 0.5 turns, and it affects everyone... That's why I stopped using the Mage's Sign....