Maps cause extreme FPS drops when loading large bodies of water

I have a MASSIVE square of water on my superflat world, made in a much older version using MCEdit. When I try to map my world, the map makes the game drop FPS, then eventually become completely unresponsive when trying to map that part of the world. The rest of the world is fine. Also, by unresponsive I mean I can't do anything, but the game is running fine.