To reproduce this error, create a page in the standard editor with 200 copies of
<source lang="xml" enclose="none"><Test></source>
and click "Preview"
The preview is never rendered, it just displays "Loading..."
However, with 199 copies it renders, although it takes a while - over 60 seconds.
Since this a round number, I suspect it is a threshold parameter. Given the length of time required for the preview it is probably not worth it to make it adjustable. But having the Preview just hangup is not a good solution. It would be better if the preview terminated with an error message rather than running indefinitely.
Note that it is still possible to save the edit, and the page itself renders fine. We are working around this by editing only by section, where the number of syntax highlight calls per section is less than the threshold.