when you cast a spell, the spell happens somewhere halfway through the animation. The frame where the spell happens in the action frame. So there are "casting frames before the action frame" and "casting frames after the action frame"
i think the delay of foh starts at the action frame. Which means the actual time between 2 foh is the delay of foh + the "casting frames before the action frame"
if that's the case, then reducing the "casting frames before the action frame" sould increase your final amount of foh/second
at 75fcr you cast in 10frames with action frame=7 so you foh every 25+7=32frames
at 86fcr you still cast in 10frames but action frame = 6 so you foh every 25+6=31frames
So it's not just about getting released quicker from your cast
http://imgur.com/Qy0QbI haven't tested that, it's just how I think it should work after having thought about it for 5min. Also I'm not even familiar with the action frame thing. I may be completely wrong.