With a family class/spousal sponsorship application you are supposed to do the medical before you submit the application, and include proof of having completed it with the ap. Same thing for the criminal clearances (police no arrest reports). If you don't do this, the application could be returned to your sponsor from CPC-Mississauga - or, if it actually gets to the overseas visa office, they will write to you and demand proof that you have completed the medical and criminal clearance requirements. They will give you a certain amount of time to submit this and they will not do anything as far as processing your application until after that time period is completed. In other words, say you don't include proof of your medical exam with your application and they send you a letter requesting it. They give you 90 days to get the required proof to them. You send the proof within 30 days - but they're not going to even look at your file for 90 days . . . so you've delayed your processing by 90 days just because you didn't do the medical before you submitted your application.
Get your medical done first - and your criminal clearances - and submit proof of both with the application.