When I cast "Decay" from the Cabalist tree it seems that it either doesn't cast (most times) or I press the hotkey, which is 4, and I wait 3 seconds for it to go off.

Only 'channeling' spells work like this.

My ping is about 70 and I'm using a fiberoptic ISP - my other casting spells, DoTs all work fine, it's only an issue with channeling spells and it's getting pretty annoying.