Why does my screen refresh slowly?

The Super Game Boy was the successor to Intelligent Systems' Wide Boy 2 (which connected to the Famicom or NES). One difference between the Wide Boy and the Super Game Boy is that the former did not use any part of the Famicom/NES other than the video memory. Even the controller (a single Famicom controller) was hardwired directly into the Wide Boy. The Wide Boy would continue running even if the reset button were held down on the Famicom/NES. The Game Boy had twice as many tiles as could fit in the Famicom/NES's video memory, so the Wide Boy had to refresh the Famicom/NES's video memory halfway down the screen.