We just tested a bit with GMSpherix if attack speed is affected by the dexterity you have.
Here are the results.
Weapon used: G15 bow
Target: TTdummy
Dexterity: 1
Time: 10s
Times attacked: 14+-1
Weapon used: G15 bow
Target: TTdummy
Dexterity: 760
Time: 10s
Times attacked: 14+-1
It is safe to conclude that dexterity alone does not affect attack speed or if it does you can barely notice it
Attack speed is constant all across the board. The only thing that affects your attack speed is your communication with your computer to the server and the server to your computer.