Steve Summit wrote:
On the subject of diff efficiency, which button is more expensive, Show Preview or Show Changes? I have an ultraparanoid bot which "presses" one or both before actually submitting, and it occurs to me that it would be marginally easier on the servers if the bot used the cheaper one first. (Though of course only if substantial numbers of its doublechecks failed, which they typically don't.)
For whole pages, generating a diff takes about 2ms plus 40ms to load the text to diff against, rendering a page takes about 800ms. So it's cheaper to use "show changes" by a factor of 20. In fact, we've been considering suppressing the "current version" display from diff pages by default, especially for large pages, since rendering the current version constitutes the vast majority of the time it takes to generate those pages. These figures are averages based on latest profiling, some pages take significantly longer than 800ms.
-- Tim Starling