Variable Attack Speed
The first thing that affects attack speed is the number of the attack cooldown. While I don't know how is it proportionate to the speed of attacking, I realized that the lower the attack cooldown, the faster the speed.
Next, is the attack style. Does it throw a ball, hurl itself to attack, or just hit from afar? From what I have seen, the speed order is something like this:
Kadabra's Confusion > Pikachu's Electro Ball > Mankey's Fury Swipes
Hits right away (Special Attack) > Shoots Projectile (Special Attack) > Launches itself at the opponent (Physical Attack)
Instant attack > Attack hits upon impact > Attack begins after landing
So, if speed is of the essence, first check out its attack cooldown, followed by its attack style.