[video=youtube;kokIAHuXYls]https://www.youtube.com/watch?v=kokIAHuXYls[/video]
[video=youtube;j6OeaNEy9Uk]https://www.youtube.com/watch?v=j6OeaNEy9Uk[/video]
Nowhere does it say that 1 second = 1 charge instead of the amount of time itself being the charge counter for damage calculation.
By lowering charge time you're stacking less charges. So basically charge half the time, do double the damage and break even.
However, now you can only stack 7.5 second of charges instead of 15 second which means the skill will be ready in half the time but only get half as much damage from charge stacking.