If you had a weapon that has normal attack speed....what would the % of IAS needed to increase the weapon to fast or even very fast? What would be the best way to increase the attack speed of a weapon?
The labels "fast" and "very fast" are rough guides at best. They are also not consistent between different char classes as FC mentioned. They actually refer to lengths of attack animations (every time you hit something, the attack animation plays, and you can't hit again until it is finished). What IAS does is speed the animations up, by reducing the number of frames they play for.
I know the game might sometimes displays "fast" on a weapon that is normally "normal" but has some IAS. What that means is that if there is no other IAS involved, then on a normal attack your char will swing it about as fast as a weapon with a fast animation and no IAS. However, it isn't exactly the same. A 12 frame animation sped up to 10 frames is generally not as good as a naturally 10 frame animation. This is because (a) there are caps on how much you can speed something up, and (b) IAS has diminishing returns, so the more of it you already have the more you need to increase the effect.
This is all a bit complicated, I know. The simple answer is "some" and "a lot". You can't really quantify it without getting into the complex details.