Note: the following analysis assumes that Focused Attacks winds up as a 100% chance for 1 energy rather than a 100% chance for 3 energy, which seems like the likely conclusion based on today's beta patch. If that's not the case, points 1 and 3 remain valid, but point 2 - and the exact numbers - go out the window. But as point 3 is the major surprising part (at least to me), I'm not going to worry about that too much.
So, as it turns out, the question of weapon speed for Mutilate builds is... much messier than we thought. The whole characterization appears to be more or less as follows:
- At weapons of equal damage, faster is better
- Higher DPS weapons are usually better, even if they're a bit slower
- If you have one slow weapon and one fast weapon, the slow weapon should go in your offhand
If you're anything like me, the first two of those seem at least plausible if not exactly obvious, and the third one seems utterly insane. But having thought about it for a while, I'm pretty sure it's right. Allow me to explain why.
Let us initially posit that we are using two weapons of equal DPS of equal speeds. We make the same number of white attacks with each hand, and the same number of Mutilate attacks with each hand; however, we additionally launch a finishing move every 10 seconds or so, which hits with only the MH. Thus, we have a few more MH attacks than OH attacks, so we put IP MH and DP OH to get a few extra IP procs (as DP is far less sensitive damagewise to increasing the number of procs).
So, that's simple enough. Now, what happens if we have two weapons of unequal speed but equal damage? Well, regardless of which way we assign them, we have the same number of FA procs, and the same amount of white damage. So the only differences are going to be Mutilate damage and Poison damage. At this point infusing some actual numbers into the equation might be useful. So lets posit that one dagger is 1.4 speed, and the other is 1.8 speed, and both are 150 DPS. This means that the average damage of the first is 210, and of the second is 270. Since all Mutilate modifiers apply to each hand equally, the primary difference damagewise is going to be which of those values is subject to the OH penalty - that is, whether the base damage is going be 1*210 + .75*280, or 1*280+.75*210. When you multiply this out, it works out to about 18 base damage. If we then apply Opportunity, Find Weakness, the poison damage bonus, HFB, Crit Rate, Armor Mitigation, and everything else, this 18 damage difference gets inflated to up around 70. So each Mutilate will hit for about an extra 70 damage if you put the slow dagger MH; you get about 1 Mutilate every 6 seconds, so that works out to be about 11 DPS.
On the other hand, whichever hand has the fast dagger is going to be the one hitting more (for a speed difference this large, anyway; I'm not going to omit this for clarity, but details are available upon request). Thus, we put IP on the fast weapon, and DP on the slow weapon, and regardless of which hands those happen to be, we'll get the same number of IP and DP procs from white attacks and Mutilates. But if we have the fast dagger MH, we will *additionally* get the IP poison procs. So how much damage do we actually gain from IP poison procs?
Well, we launch a finisher about once every 10 seconds; each finisher has a 30% chance to proc IP, doing about 1500 damage. So the difference in IP damage is about 1500 * .3 / 10 = 45 DPS, while the damage gain on Deadly Poison from that extra attack is more like 5 DPS. So by putting a fast weapon in the MH, we gain a free 40 DPS in poison procs, give up 11 DPS in Mutilate damage, and thus come out 30 DPS ahead relative to putting the slow weapon MH.
So, that's weapons of equal speed - what happens when the weapons aren't the same speed? Short answer: it takes a pretty big weapon damage difference to make up that 30 DPS gap. For instance, using Webbed Death (156/1.4) and Sinister Revenge (171/1.8), I still show a 15 DPS advantage to having the faster weapon in the MH, even given the 15 base DPS difference. Hence, you'd probably need a weapon that was ~30 DPS ahead in addition to being slower before it made sense to MH it.