Name-tagged mobs causing large FPS drops.

A zombie pigman wandered into my base so naturally I decided to keep him and called him Zisteau.

Now, here's the issue. When I look at Zisteau, especially when standing close to him, the FPS begins to drop sharply.

Observing the pie chart on the debug screen (shift+F3), it is clear that something is wrong. The render portion grows larger and larger the longer I look at him. As soon as I move my cursor away, everything goes back to normal.

I will try and record a video of this happening soon.