A diddler is someone who is constantly getting side tracked at the most inappropriate times. They're perceived as slow when trying to get things done and their foregoings always end up conflicted and confusing.
Not associated with 'diddle or diddling' but these terms can be used to further diddler. You can be 'diddling' or 'diddle' around in your slow processes.
Example: 'About time you got here Johnny... You said you'd be here an hour ago.'