First, as was pointed out by Mr. Ehsassi earlier and by others, the number of prosecutions is so minimal that it has no effect on efficiency.
On the other hand, you don't give prosecutors discretion when it comes to crimes that are universally considered to be jus cogens crimes. Canada has signed on to the treaties, including the genocide convention, which by itself prohibits the promotion and glorification of genocide.
Leaving it to a prosecutor is like leaving to the Nuremberg prosecutors to say, “You know what, let's call this a summary conviction offence.” Can you imagine the prisoners in the dock being told, “Don't worry”? It's not the penalty. It's the title of summary conviction.