Originally Posted by
Rumstein
Mind Flay ticks at 0, 0.75, 1.5, 2.25, 3 seconds, or GCD normalized it ticks at 0, 0.5 1, 1.5, 2 GCDs. Because it deals 60% of it's damage and insanity gain by the end of the first GCD and 40% in the 2nd, the first GCD is better for Mind Flay than Mind Spike. At latter levels of haste (GCD capped +), Mind Flay will deal more damage per GCD as the channel will last for less than 2 GCDs while Mind Spike uses 2 GCDs entirely.
Imagine being at 100% Haste (Can reach that with just bloodlust and good gear). Mind Flay takes 1.5 seconds for a full channel, while a GCD is capped at 1 second (outside of VF). For slightly more damage than a full Mind Flay channel, Mind Spike can be used twice, but that will take 2 GCDs or 2 seconds. In 3 seconds you can cast 3 Mind Spikes, or 2 entire Mind Flays - which deals more damage than Spike.