Is "+attack speed" more beneficial to fast weapons?

Short Answer: No.

Because attack speed is based on attacks per second and additive, a bonus to attack speed will affect fast and slow weapons equally.

Long Answer: Noooooooooooooooooooooooooooooooooooooo...

Some basic maths can be used to demonstrate this.

Consider the following:

• A very slow weapon that hits very hard ("Weapon A")
• A very fast weapon that hits very weakly ("Weapon B")
• +100% attack speed halves the attack interval (that is to say, you get twice as many attacks per second as you did before).

                         Damage     Attacks       Damage   
                         Dealt     per Second    per Second

Weapon A                  100 HP  *   .5    =   50 DPS
Weapon A +100% atk speed  100 HP  *    1    =  100 DPS

Weapon B                    1 HP  *   50    =   50 DPS
Weapon B +100% atk speed    1 HP  *   100   =  100 DPS

As you can see, each weapon benefits equally from the attack speed boost, regardless of the starting attack interval.

This works for any value of attack speed, because it stacks **additively*. To wit, +20% attack speed plus +50% attack speed = +70% attack speed. The above example holds true for any value of attack speed; as long as your math is right, you'll get the same increase to DPS independent of your starting attacks per second).

There's some argument to be made back and forth about the benefits of attack speed for things other than your "Auto-Attack" damage, but they'll still have the same relative comparison after the attack speed has been applied.