Originally Posted by Malconstant

There seems to be some confusion. It has been stated that the effect spell haste rating has on overall DPS is linear, and yet follow up responses seem to suggest a belief that there is a diminishing return in overall DPS. This is simply not the case. Even though there is a diminishing rate of decreased casting speed with spell haste rating, the increase in overall dps is constant, i.e. linear.

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That in and of itself is unique to haste rating. The result is that haste rating scales with gear better than any other stat. For instance, once the lucky warlock has attained 2000 spell damage, the effect of gaining an additional 1 spell damage has a smaller effect on overall %DPS than when the warlock only had 1800 spell damage. For spell haste rating, however, even when the warlock has 1569 spell haste rating, the percentage increase in DPS from of gaining one more spell haste rating is equal to the percentage increase in DPS going from 18 to 19 spell haste rating, etc.. Of course, this does not mean that getting a large spell haste rating will always result in the most DPS, only that its effect on dps is linear.