Processing times will differ by country and can vary for many reasons. Usually waiting in the queue is the longest part of processing for anyone's application. Other than queuing, things that will slow down the actual processing are typically bad immigration history, criminal convictions, previously visa refusals, and if requested by an ECO - waiting on additional documents.
I believe recent applications from Canada haven't been taking very long - looking at the UKVI site for historical processing times, your application should definitely be processed within 60 days for non-priority status. 12 weeks is only a guide, nobody can accurately predict the processing time. Paying extra for priority processing only puts your application in front of non-priority applications. But it doesn't guarantee that your visa will be processed in a certain amount of time.
I am from the US and my visa was processed with priority service, I had a decision after 8 working days. On various forums I have even seen some applicants who opted to not pay for priority processing, and they received an answer in as little as 5 working days. This is very rare, and not something anyone should anticipate to see every time!
The ECO wants an idea of when you'd like to travel. Your proposed date of travel does not influence how quickly your application is processed. If your application isn't decided by your proposed date then your 30 day travel vignette will reflect a later date. According to the home office, you can apply no more than 3 months before your intended date of travel. So if you wanted your visa to start as soon as it was issued, you could put the date of application as your travel date or soon after. Sometimes they will ignore what your proposed travel date was entirely and make the visa valid from the date of issue. This is why you are highly advised to not purchase your flight tickets until a decision has been made and you've received your passport back.
We've heard various dates given to applicants when the 30-day window is assigned to them. Some people have found that their 30-day period begins with their desired travel date, and others have said that it ends on the day. So unfortunately you have to wait and see what they have granted when your passport is returned to you. My proposed travel date was two weeks into my 30 day travel window – but this did not inconvenience me.
Wishing you the best.