Fiddles fear has about a 0.25 second delay
The easiest way to prove this is a problem is to use your Q immediately before a champion becomes un-targetable. Example:
- Fiora runs up to ulti
- Q Fiora (Fiddles animation goes off)
- Fiora ulti's
- Fiora is silenced WHILE ULTI'ing
Because her ulti still goes off she waits out the silence in utter safety and still does full damage during the fight, meanwhile Fiddle's fear goes on cooldown and is wasted.
This can also be seen with Fizz's Playful, Zhonya's hourglass, or something as simple as flash (fear someone immediately before they flash over a wall and they land on the other side of the wall fear'd). You do not need to have vision of them when the fear takes effect, only when you initially cast.
I have experienced this regardless of build or items at all points during the game. I don't believe it can be attributed to lag as, if I lagged, I would expect the enemy to not get silenced at all (as they were out of range when the ability "actually" went off).
I've also noticed that if Fiddle's becomes stunned, silenced, suppressed, or otherwise during this 0.25 second window his fear will go on cooldown but have no effect.