Alacrity in an attribute that increases the cast times and channel times of spells. It is important to note that it does not affect the GCD nor does it increase the speed at which healing over time (HOTS) and damage over time (DOTS) tick.
For example: A player with a 400 alacrity rating will notice that a spell with a 1.5s cast time now casts at 1.32s instead. Assuming you are firing at a constant rate, this can interpreted as a damage per second (DPS) gain of about 13.2%. Not bad for a small investment of 400 points.
Thanks to the people at sithwarrior.com we know that alacrity can be calculated using this formula:
NCT = New Cast Time
CT = Cast Time
AFS = Alacrity From skills (such as Mental alacrity)
Plotting this formula (at level 50) from 0 to 1000 alacrity produces this graph:
Note that the dps gain from alacrity is independent of casts time. Alacrity also suffers from diminishing returns. From 0 to 200 alacrity on a 1.5s cast time spell will net you 0.1 seconds while going from 200 to 400 will only net you 0.08 seconds.
Jedi Sages get an ability called Mental Alacrity with increases alacrity by 20%. Plotting the same graph with the alacrity buff from this ability produces this graph:
Experienced raiders will realize what a great skill mental alacrity really is. There are some good boss fights where it can be used efficiently. For example: After igniting G4-B3 droid (the 4th boss in Karagga’s Palace).
While alacrity is a great pve skill, it’s very difficult to use efficiently in pvp unless you are a healer. Because of the the binary nature of pvp (either you’re getting hit or not), this will mean that you are probably better of stacking surge and using an alacrity adrenal.
Speaking of surge, how does an equal amount of alacrity compare with an equal amount of surge? From my surge blog we know 400 surge provides a critical damage bonus of 190%. Comparing that with 400 alacrity:
400 surge will provide a dps boost:
30% crit chance; (0.30 * 1.9 + 0.65 * 1)/(0.30 * 1.5 + 0.65 * 1) = 1.109 or a 10.9% gain
35% crit chance; (0.35 * 1.9 + 0.65 * 1)/(0.35 * 1.5 + 0.65 * 1) = 1.119 or a 11.9% gain
40% crit chance; (0.40 * 1.9 + 0.65 * 1)/(0.40 * 1.5 + 0.65 * 1) = 1.128 or a 12.8% gain
400 alacrity will provide a dps boost:
3s/2.64964976496s = 1.1322.s or a 13.22% gain
Alacrity is the slightly better. I would still suggest stacking surge from 350 to about 400 as mentioned in my surge blog. While there is a slight advantage to using alacrity, the advantages are not large enough to warrant some major changes to your gear setup (for example dots not scaling). If you are going for a pure dps build like 31 telekenetics, then it is definitely worth stacking.