The Speed is something that I have been trying to get my head around. I do not have any facts or lengthy tests that I've done yet either. The way that I have observed speed behave in the game reminds me of the "time gauge" from Final Fantasy VII. The time gauge for all monsters and PC's fill a base rate of X and when full they get to act. Speed just makes the time gauge fill at X + Speed bonus + Haste + Slow (or - Slow depending on the maths for it.) I have taken to getting as much augment slot armor I can wear that is not - speed and filling it with speed runes.
In my observations of speed I have seen a PC act 3 times consecutively. I have had 2 PC's take 9 consecutively actions between them as well. In a boss fight my four PC's had 5 stacks of haste and +10 speed on chest, head, and neck armor. Together the PC's had 15 attacks before the boss got to have its first attack. The boss fight was