As has been noted previously, typing MathJax in answers can significantly cause the page to lag. On my Mac this lag is quite severe, even with only minimal MathJax introduced, making it really irritating (and generally makes me think 'lets just take a screenshot').
On long posts, a
hide preview link appears (I noticed this when I've been trying to copy and paste @Orthocresol's character tables ), however it it doesn't show up by default.
Please let us have this feature enabled by default so that the preview can be hidden immediately once one begins typing a post.