Theorycrafting 101: Haste Buff Stacking
by, 26-08-2012 at 11:02 PM (16814 Views)
One of my long standing buff stacking rules has been to avoid stacking similar secondary stat buffs, ie: donít combine two crit buffs, or two haste buffs. This sort of rule doesnít apply to primary stat (Agi, Int, Str) or their power types (Attack/Spell). Iíve had this rule for a number of years now, so I decided to review the math on it, both to double check it and explain the reasoning behind it.
The methodology was fairly straightforward. Work out the cast time for a spell (in this case, Lightning Bolt) under various haste conditions (base haste 0-25% with Bloodlust, Elemental Mastery and then both Bloodlust & Elemental Mastery at the same time). I also ran the same comparison for the Global Cooldown as this will hit its haste cap at 50% which is fairly easy to hit.
Before we start, I should clarify something about how haste buffs work. The effect of haste lasts longer than the duration of the buff, as spell cast times are calculated at the start of the cast rather than at the end, so any time you want to work out the number of spells affected you divide the buff time by the new cast time and then round up to the nearest whole number.
The cast results were rouhgly as I expected, with a 0.5% increase in the number of casts over 6 minutes when under the ďhardĒ haste cap for Lightning Bolt of 100% during the stacked buffs.
So in this case, at 10% base haste Bloostlust adds 7 extra casts vs the 11 that Elemental Mastery adds (which is 4 per use, less one cast at the end). When stacking buffs, it adds an extra 4 casts to the Bloodlust time, but you lose 4 extra casts from the third Elemental Mastery use. So now weíve got 7 + 11 = 18 and (7 + 4) + (11 - 4) = 18 comparing non-stacked with stacked. The other part is that the non-hasted casts for stacking is 153 vs 142 for non-stacking. This difference is made up for by the third Elemental Mastery cast covering that 11 cast difference.
Things start to change when the stacked haste total passes our hard haste cap of 100%. The slight difference is wiped out, and then the total haste creeps past 110% thereís a slight gain on the non-stacked side.
So, assuming youíre just casting spells like this it doesnít matter which way you go with using your haste buffs, but what happens when you use instant casts? Itís somewhat the reverse, as the Global Cooldown is only 1.5 seconds long so itís already capped when you combine two 30% buffs or one buff with 16% base haste.
Combining buffs results in 3-5 fewer GCD uses in 6 minutes, so given the fairly neutral overall picture for spell casts, itís better to avoid stacking your haste buffs. Of course, in certain situations itís preferable, but thatís usually an exception like a critical burn phase.
If you want to look at the spreadsheet I used for this, itís located here. Just be aware that itís not pretty, so it may be confusing for some.
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