REFER TO THE FIVE POINTS BY CIC TO DECREASE UR PROCESSING TIME: http://www.cic.gc.ca/english/resources/publications/delay.asp
And as per the information, it is better to do medical upfront
You can avoid delays by getting your medical examination done quickly and submitting it with your application. It is not mandatory to complete a medical examination when you submit your application. However, doing so will result in faster processing. For more information about who needs a medical examination, please visit www.cic.gc.ca/english/information/medical/index.asp.
Applying online and getting assistance for visa application from VAC are better choices than preparing offline applications urselves and mailing them
VACs make the Canadian visa process more convenient for visa applicants around the world by providing additional points of service. VACs make sure applications are complete and that they meet all of CIC’s requirements by pre-screening applications. If a VAC is located in your country, consider using its services.