I created a full size map (4/4) and was using the /tp command to try to quickly fill in the map. After doing this for a while I started to notice some lag, so I checked the debug screen, and I was at 99% memory usage. When I tried to shut down Minecraft, it froze and took several minutes to exit.

I increased the memory to 4GB and tried to recreate the problem. I used /tp in the same pattern and was able to see the memory slowly climb as I teleported around. It got memory usage up to 30% before I quit, and I expect it would have kept climbing as I kept using /tp. Perhaps this is related to teleporting into ungenerated chunks while the chunks I was in were still gnenerating or rendering.

When mapping the "normal" way (flying or walking) I did not see the issue. I did this in single player, so I don't know if this is a server or client issue.

I started a (0, 0) and used the following commands to teleport into ungenerated chunks.

/tp ~ ~ ~128
/tp ~128 ~ ~-2048

I used the first command multiple times until I reached the bottom of a map, and then used the second command to go back to the top of the map, and repeated.