Couple of questions... will help us help you here with some more info...
First, what road were you on? How fast was traffic around you going? What were the road conditions like? Finally, if you were on a multi-lane road, which lane were you in when the cop tagged you? All of that is important. There's no specific statement of "too slow" in the HTA. Well, actually, there is. The section you got charged under is this:
132. (1) No motor vehicle shall be driven on a highway at such a slow rate of speed as to impede or block the normal and reasonable movement of traffic thereon except when the slow rate of speed is necessary for safe operation having regard to all the circumstances.
Speed limit isn't in there. It is included in the wording of laws against driving too slowly in British Columbia for example, but not Ontario. Quebec, of course, has MINIMUM speed limits. So do some American states, like Michigan. In Ontario, you could, in theory, get charged for unnecessary slow driving even if you're driving the speed limit. 
10 under limit, keeping right except to pass, or just driving on a rural highway (like Highway 17), ticket is easily beaten. If you were going 80 in the left lane of the DVP with traffic ripping by you in the centre and right lanes... that's a whole different ballgame. Regardless, be sure to get disclosure to see the evidence against you.
Sounds like you already had some sort of a "First Attendance" meeting, from your post under General Talk. Crown didn't want to drop the charge, no surprise. They only want a guilty plea out of it, so you did the right thing by carrying forward if you wanted to avoid the demerit points.