According to the description of Deathfire Touh (Keystone in Ferocity), the damage dealt is dealt faster when using AoE and DoT rather than single target abilities. Text says (please note that I make an exaple damage) "... to take magic damage over 4 seconds. Damage dealt is <formula comes here>. Deathfire touch's duration is reduced for: Area of Effect spells: 2 seconds, Damage over Time spells: 1 second".
If we make an example, we go with 100 damage:
single target spell would deal 100 damage over 4 seconds => 25 damage/sec
Area of effets would deal 100 damage over 2 seconds => 50 dmg/sec
Damage over time would deal 100 damage over 1 second => 100 damage/sec
giving a champion like Malzahar the power to deal 4 (or whatever amount or seconds E lasts) x100 damage using E and further 2.5x 100 using R
Are you sure that you didn't mean to write "... to take magic damage every second for 4 seconds...."?
If not then please try to explain me why in plain english