While editing this answer on Cryptography Beta, I discovered that MathJax re-renders all equations in the preview every time a character gets added to or removed from the text area (whether this affects an equation or not).
This doesn't only seem unnecessary, it makes typing rather laggy, causes my CPU usage to spike to 100% on one core if I type fast enough and caused my Chrome tab to freeze on multiple occasions. After losing parts of my edit in the second time, I decided to edit the answer in a text editor and paste in the text area later.
Even scrolling (holding down →) can get painfully slow from time to time, although I didn't manage to find out what exactly triggers this behavior.
This also affects other sites that use MathJax (e.g., Mathematics).
In case it matters, I'm using Google Chrome 23.0.1271.97 on Ubuntu 12.10.