NOTE: This post is based 5.2/5.3 mechanics when all RPPM items scaled with haste. This is no longer true. For more up to date discussion of haste vs. mastery for assassination rogues refer to my 5.4 Rogue Changes Guide or better yet ShadowCraft.
Haste vs. Mastery gearing for assassination has become one of those questions where there is a lot of misinformation flying, some of it my fault, around so my goal here is to set the record a bit straighter.
Lets begin with the common assertion that haste increases RNG and people gearing haste will win by a large margin with good RNG or be hurt substantially by bad RNG. This assertion is false, the effect of haste is to mitigate the effect of RNG from RPPM not increase it. The simplest way to show this would simply to show you SimCraft’s dps variance calculations (see below) and leave it at that but lets go a further and look at why this is the case.
One of the tricky things about considering trinkets is that trinket procs are discrete, having 6.7 trinket procs isn’t any better than 6.3 because there is no such thing as .3 or .7 trinket procs. Below is a graph of the expected number of procs over a 10 minute fight as a function of haste for Bad Juju and Renataki’s Soul Charm.
As expected haste increases the number of procs over the fight however there are large haste ranges where additional haste will not increase the expected proc rate. Given this chart a naive player might aim to reduce their haste as low as possible while still getting an extra proc much like casters do with haste breakpoints. However RPPM as we know has a fair degree of variance so lets look at that as well. The graph below looks at good and bad RNG variances of RSC, in good RNG you have a 10% higher proc chance and in bad RNG is 10% lower proc chance.
Again the graph looks as expected, a higher proc chance means more procs. The important thing to note is there are haste ranges where under bad RNG you do not lose a proc. Obviously this is somewhat simplistic since RNG doesn’t just produce exactly +/-10% variances but the principle that haste insulates you from bad RNG is clear. It is true that haste can also give you RNG dps increase, on the above graph points where the good RNG has more procs then the expected but this doesn’t make a spec more reliant on RNG.
I’m talking only about trinkets and not the legendary metagem in this analysis because the legendary metagem’s higher proc rate makes major RNG swings less likely. While there will almost certainly be some variance in metagem procs from pull to pull the magnitude of an additional proc is substantially less than that of a trinket to the point where we can use a linear rather than discrete approximate.
Another common assertion about haste is, haste is more valuable on a Patchwerk fight and less valuable on mobile fights with time off target or a multi-target fight. This assertion is true although it’s nowhere near as important as its proponents suggest.
The logic goes, during time off target rupture and deadly poison are still ticking, and you are gaining energy but not much else is happening. A mastery build will do more damage in this case compared to a haste build because deadly poison and venomous wounds damage scale with mastery and not with haste. Further a rogue forced off target can use this time to pool energy which effectively eliminates the special attack damage lost so long as the rogue does not energy cap. Once you energy cap everything that damage is lost. Again this benefits the mastery rogue because they can wait longer to energy cap. Finally in a multi-target environment you can only melee swing on one target at a time so the additional mastery will increase the value of deadly poison and venomous wounds procs.
To look at this in a more structured way, here below are the SimCraft outputs of a haste and mastery build doing a hypothetical encounter where the player must move 20 yards every 35 seconds and 20 yards of movement every 20 seconds. These movements do not use Shadowstep so broadly represent a wide variety of off target events.
The above shows its certainly true that mastery build lose less damage from time target however the facts of current raids makes that less relevant. The second movement example, 20 yards of movement every 20 seconds is a more extreme movement example than most if not all encounters this tier. Realistically for most fights this tier, and last tier for that matter, the melee role has been very similar to Patchwork. Multi-target fights where mastery does have an advantage are somewhat more common. I’m not familiar enough with SimCraft action list syntax to be confident in creating a multi-target action list so I won’t use SimCraft to look at this question more empirically however based on previous results I doubt the results are a clear slam dunk for mastery builds.
This brings us to the final question of, how should I gear and the answer, like most answers worth heading, is it’s complicated. Evaluating the exact conditions of a given encounter are complex, even with SimCraft, and regemming/reforging for every encounter is more min-maxing than most players will do. As demonstrated above the difference between haste and mastery on most realistic encounters is quite negligible. For players who do want to minmax to the nth degree I believe the data above shows that haste is not being systematically overvalued by our theorycrafting tools and balanced haste and mastery or possibly slightly haste favoring appear optimal.
A final caveat, this analysis is focused on stat priorities with multiple RPPM trinkets and the legendary metagem where ShadowCraft and SimCraft show haste as more valuable than mastery. At lower gear levels/in the absence of multiple RPPM items where ShadowCraft and SimCraft recommend mastery over haste you should follow that advice.