Is it worth checking whether a file *should* be highlighted?

[chriskrycho]

To-benchmark: what are the performance characteristics of searching a large string blob for the characters <pre vs. doing the full XML event stream of the files? The potential performance tradeoff is: it would be easy to eliminate a lot of files which simply don't need to be syntax highlighted (because they don't have any <pre><code>...</code></pre> blocks at all). But it's all down to that single tradeoff: string searching vs. XML streaming.