Typically impeding traffic. Notice it does not reference the speed limit, but the normal and reasonable movement. By extension, driving at the maximum legal speed limit is below the normal and reasonable movement of traffic.
According to the Ontario Highway Driving Act, section 132: “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.”