there is no speed difference. the interval between strafe shots is shorter than DS shots because strafe has a longer animation duration.
if the arrows go flying forever, this is what you would see...
Code:
<empty space><strafe><empty space><strafe>
< empty space ><DS>< empty space ><DS>
you see, it is an illusion that DS is slower... the longer empty space between DS is due to the fact that DS is a very very simple animation of just 2 arrows flying off the bow while strafe has the initial blue rim followed by 4 arrows stretched out that flies from slightly behind the shooter.