Casting Speed

General

The animation of all spells¹ is split into two part, 'startup' and 'casting'. Depending on the player's dexterity and spell in question, a certain number of frames at the end of the startup animation may be skipped, shortening the duration of the attack as a whole and of the time it takes for the attack's hit to occur.
Although this naturally makes attacks safer and increases DPS, the effect on PvE is negligible, while it is indispensable for most PvP builds and is partially responsible for the dominance of Pyromancy in the game's meta.

Mechanic

The startup animation ends when the earlier of two events occurs:

• A certain number of frames pass. This is a hard-cutoff, and represents the casting speed at low levels of dex.
• The end of a specific time-window is reached. For all spells that can be shortened, this will occur at all levels of dex beyond a certain freshold.

This time-window is placed near the end of the startup animation, has a set length which it maintains while having 20 or less dexterity, has a duration of 0 frames (the animation ends immediately when the window would start) at 45 dex or above, and in between shortens linearly. In mathematical form:

(1)

\begin{align} \textrm{ShorteningWindowDuration} = \begin{Bmatrix} \textrm{MaxWindowDuration} &\textrm{Dex}\leq20\\ \textrm{MaxWindowDuration}\cdot\frac{45-\textrm{Dex}}{25} &20\leq\textrm{Dex}\leq45\\ 0 &45\leq\textrm{Dex}\\ \end{Bmatrix} \end{align}

One possible method of modelling the entire behavior would be to divide the startup animation into: pre-Active-Window, Active-Window and Passive-Window.
In this way, the pAW would represent the duration at 45 dex as well as the start of the window, pAW+AW represents the duration at low dexterity, and AW+PW the duration of the entire window.

Startup Animation durations

Animation ID2	Example	pAW	AW	PW
6200	Soul Arrow	10	5	47
6201	Magic Weapon	39	-	-
6202	Hush	29	-	-
6203	Heal	54	-	-
6204	Fireball	18	12	22
6205	Power Within	29	-	-
6207	Combustion	9	6	20
6208	Poison Mist	39	-	-
6209	Firestorm	30	20	27
6210	Undead Rapport	44	-	-
6211	Chameleon	9	-	-
6214	Sunlight Blade	49	-	-
6215	Gravelord Sword Dance	54	-	-
6316	Magic Shield	35	-	-
6217	Emit Force	39	-	-
6218	Lightning Spear	30	20	28
6219	Cast Light	39	-	-
6220	Soothing Sunlight	54	-	-
6222	Force	9	-	-
6224	Heavy Soul Arrow	30	20	27
6225	Fire Surge	10	10	47
6226	Fire Whip	10	10	47

• The startup animation for all left-handed casting starts with 62, all corresponding right-handed animations start with 63. These values are all identical between the two, with the exceptions:
  • Fire Surge - When used with the right-hand, it does not have a shortening window at all and is not sped up by dexterity.
  • Magic Weapon - Does not have a corresponding animation.
  • Magic Shield - Startup of Second Chance from Demon's Souls, naturally unused.
• Interactive calculator (sheet)

Footnotes

1. Except Fire Surge, which has a looping attack animation and a release animation

2. Any skipped numbers are used for non-spell related animation, none of which are shortened in the manner covered in this article