How do mages deal so much auto attack damage to turrets without any bonus AD?

I've noticed this that even without any Dragon buffs, mages somehow chunk turrets really fast in late game. And since the mastery that gave you bonus on hit magic damage that scaled with your AP was removed with the new season, I don't understand how they deal so much damage to turrets, because auto attacking turrets with ~100 Attack damage should take forever and a speed of less than 1.00. Anyone care to explain?
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