I think you need to prepare quite some other documents in addition to the invitation letter and certificate of enrollment. You should follow the CIC instructions, especially the document checklist, to prepare your documents. It is very important to provide the necessary documents as complete as possible.
As for medical examination, if you only invite your parents over for a short time, say 1 month, most likely they won't need the medical examination. However, if they are very old, they will need medical examination no matter what.
If your parents go to the beijing or hongkong and submit the application themselves, they can get the results the same day (of course, only if they are not required to pass medical examination). One exception is if they are residents in Beijing, then they need to wait some time for the results.
If your parents are going to submit the application to embassy in Beijing or hongkong, please go check the website of embassy in Beijing or Hongkong accordingly for specific instructions.
Hope the above information helps and good luck!