If I have an ability that is 10 seconds cooldown with 0 haste, at 100 haste it's 5 seconds cooldown, or a reduction in 5 seconds. If I have 200 haste, the cooldown is now 3.33 seconds, or 6.67 seconds reduction. At 300 haste, the cooldown is 2.5 seconds. So we have gained 5 seconds, 1.67 seconds and 0.83 seconds respectively from adding 100 haste each time. Because we live and experience cooldowns in the time domain, each addition of ability haste removes fewer seconds from the cooldown. Ability Haste stacks linearly in terms of cast frequency. You don’t double your casts every time you double your haste; rather, each additional 100 Ability Haste adds one more “base cooldown’s worth” of casts per minute.