First go to your world options in the lobby and set the skills to local. You can turn them off ingame, But if they are on global, The Item command will not work.
Then for each item you want to change the swing speed of, Make a line of script with this command.
"Item [name] [speed=X] [delay=X]"
Speed determines the total item swing time in seconds. Delay is the amount of that time that is spent with no animation.
For example, If you wanted a warhammer to take 0.75 seconds to swing, And have a 0.25 second delay before you can swing again, You would use this command.
"Item [TitaniumWarhammer] [speed=1] [delay=0.25]"