How long does it take to get a Canada visiting visa?

I need to attend a conference in Canada late March. Just checked, the website says that one needs to apply at least one month before departure. Anyone can tell me what is the turn around time for mailing service?
Also, in the webpage it says "If you are from the United States, Greenland or St. Pierre and Miquelon, you can apply at a Canadian port of entry." Does that mean that I, not a US resident, can do that too?
Thank you very much!