I thought changing this one would change the speed of an item...
<property name="Delay" value="1.33"/>
I may be wrong but thought it had worked for me in past. It seemed to work because when I used it on the stone axe I noticed the sound didnt match the hit after.
EDIT: I should have included that I made the change in the item.xml file and I decreased the highlighted value until I thought it worked ok for me.