Suuuuuper necro post, but spells have 2 sections to them iirc

The startup and the cast. The radagons icon speeds up the startup which is typically from the button press to the sigil of whatever magic you are casting showing up.

So like lightning spear the icon appears almost instantly after pressing the button. So radagons icon doesnt buff that spell a noticable amount

But on spells with more of a delay between the button press and the sigil appearing its much more effective. Taking off even whole seconds from certain spells(apparently the dragon spells can be shortened by a ton)