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