I was thinking about creating an extension that automatically saves a HTML version of a markdown formatted field to another custom field.

So say you have a custom field ‘article_body’ that’s formatted via markdown, when that entry is saved it’ll store the HTML formatted text in ‘article_html_body’.  Then you could just display {article_html_body} without having to call markdown each time the page is loaded.

Do you think there’d be any noticable performance gain from doing this?

The Markdown plugin isn’t considerably “slow”, so…noticeable performance gain?  Not likely, especially if you are using template caching, but it’s an interesting idea, and of course would have a measurable effect; I just doubt that it will be perceivable.  However, also consider that the time you are saving by pre-formatting the field at publish time is also increasing the memory and time fetching an additional custom field from the database.  We’re talking fractions of fractions of fractions of resources here, though.

Hmm, OK thanks Derek.  There doesn’t seem much point then.  Cheers though.