Yes, CPU usage is a minor concern. But there are ways to make it not such a big deal.
The specific case that Vautrin seems to be talking about is when somebody has a big block of Perl code that they have pasted into code blocks. In those cases, it can be hard to follow for those of us who have been spoiled with syntax highlighting. I don't think that means that all code blocks need to be highlighted all the time.
As multiple others have already suggested, if there was a link, similar to the download code link, that was basically a "View code with context markup", then the monks could use that link whenever they wanted to, but otherwise it would take up no more CPU than normal. This also solves the "not all code blocks are Perl" issue.
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Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link or
or How to display code and escape characters
are good places to start.