Sometimes it takes longer than (what I believe to be) the expected time of ~1s, and sometimes it just doesn't seem to activate at all. Please just remove it.
Let's get away from these gotchas and just make things a bit simpler. The game's already plenty hard enough without this kind of stuff getting in the way.
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With Prismatic you have so many ways to stun all champions in a single build. Although the wait on weapons is annoying lol.