Arcane Crystal

It costs you a Mana Crystal, so only do this once your mana pool is already capped and the crystals have nothing left to raise. Shimmer converts one into a permanent regeneration upgrade instead, and each character can take that trade exactly once.

The part you feel is the dead time after you stop casting. Mana normally waits a moment before it starts coming back, and this shortens that wait as well as speeding up the refill. The benefit is biggest when you fire in bursts and pause, which is how most mage fights actually play out, and smallest if you simply hold the button forever. A character who never casts anything can skip it without regret.