Dungeons and Dragons games, and ones which copy them, penalize you for wearing heavy armor on a wizard.
This isn't a Dungeons and Dragons game.
If you have the stats for something, you can use it - weapons/armor/peanuts whatever you fancy, so if your caster has a Str 10, feel free to equip that plate helm and mail armor.
Movement speed is an attribute on armor, it's true that leather armor and robes (plus equipping daggers) have movement bonuses on them while plate armor brings a movement penalty (scale armor having a lower str requirement and lower movement penalty).
But please note - like resistanes/attributes etc, you can find a sexy suit of mail/plate armor which has a blue movement attribute that has either a low movement penalty, or even a movement buff. Plus rings can have movement bonuses too, so it's possible to have a super fast Madora who zips over the battlefield.
