I tested spell speed this evening.
Fast
2.23 seconds consumed
Frames to Effect: 14
Frames to Completion: 35
Frames of Recovery: 32
AVERAGE
4.3 seconds consumed
Frames to Effect: 53
Frames to Completion: 79
Frames of Recovery: 50
SLOW
7.93 seconds consumed
Frames to Effect: 128
Frames to Completion: 132
Frames of Recovery: 106
I used naked Wood Elves with 10 Dexterity as a baseline. Recorded casting in FRAPS at 30 FPS and checked speed frame by frame in After Effects. Dissected single spells and verified with queues of 5 spells. Speed seems to vary some within categories, as fireball and burst of summer flame gave different results. There is a slight delay between when the system declares a spell has been cast and when it realizes that spell's effect, probably based on the effect's animation. I tested the same spell with 10 Dex and 20 Dex to find that the next spell occurs 30% faster, so Dex expedites casting time and recovery time, but had little effect on realization time.
For comparison, a hunting bow is 3.0 seconds consumed, 45 frames to completion, 45 frames of recovery. Fast spells are ~25% faster than hunting bows, deal ~100% more base damage and can reach a 1.4x damage multiplier (talent, bracers), so a good fast spell is akin to a hunting bow with a 2.4x damage multiplier and swift aim.
Below is my raw data pasted from notepad.
--------- FAST ---------
(DRUID summer flame)
10 frames to be told cast
13 frames to see damage
35 frames to end casting animation
32 frames to start new casting animation
//13 to damage [0.43 seconds]
35 to end [1.17 seconds]
32 to new action [1.07 seconds]
67 frames total [2.23 seconds]
(DRUID summer flame [20 Dexterity])
7 frames to be told cast
12 frames to see damage
29 frames to end casting animation
23 frames to start new casting animation
//12 to damage [0.4 seconds]
29 to end [0.97 seconds]
23 to new action [0.77 seconds]
52 frames total [1.73 seconds]
(WIZARD fireball)
14 frames to be told cast
17 frames to see damage
36 frames to end casting animation
34 frames to start new casting animation
//17 to damage [0.56 seconds]
36 to end casting animation [1.2 seconds]
34 to new action [1.13 seconds]
70 frames total [2.33 seconds]
--------- AVERAGE ---------
(DRUID blizzard)
50 frames to be told cast
53 frames to see damage
79 frames to end casting animation
50 frames to start new casting animation
//53 to damage [1.77 seconds]
79 to end [2.63 seconds]
50 to new action [1.67 seconds]
129 frames total [4.3 seconds]
--------- SLOW ---------
(DRUID returning storm)
128 frames to be told cast [4.27 seconds]
132 frames to end casting animation [4.4]
106 frames to new action [3.53]
238 frames total [7.93 seconds]
*Changed a few frames after verifying with a paused queue. Fast -1 frame, Average +3 frames.