The movement is slow because it's too difficult and complicated!
To be effective, you have to do movements CORRECTLY.
You could not do tai chi, with full and correct "fundamentals", fast!
Who don't know these and keep these in mind is not a tai chi "practitioner"
_ "Chi" focus into dan tian
_ Back straight, chest retract a little bit
_ Head is as if something is hanging it to the ceiling
_ Shoulder down, your "zhou" (the middle part of your arm) also down
_ Waist relax...
(too many character)
it is slow when you do it as an exercise ... but once the form of the movement grows on to you, you'll natural move your body when a defensive situation occurs