The Honourable Judge B. R. Fraser has other matters to attend to outside of simply working on the Tschetter sentencing. He may have a few days here and there to work on the sentencing decision exclusively but he is probably also hearing docket days and other trials between now and September. Those other matters require him to spend time, effort and thought that could otherwise be used to render the Tschetter sentence quicker.
You also have to consider that there will be a good deal of precedential value to this decision. As defence counsel have suggested, manslaughter charges are rare to non-existent for vehicle-related deaths in Canada. This will be among the first (and definitely worst) on the books in Canada. The Crown will look to this case as a new high-water mark for deaths caused by reckless/dangerous/drunk drivers and will seek to rely on it to obtain tougher sentences for people who kill others with their car. It will serve all in the legal community well for Judge Fraser to get this right.
To compare this apple to an orange, the Supreme Court in 2008 averaged about 4 months between the time an appeal was heard and the day judgment was released:
http://www.scc-csc.gc.ca/stat/html/cat5-eng.asp