Having finally run through the process of excluding one by one each piece of software involved, the culprit was TextWrangler. I'm now editing the blog posts in TextEdit, which is far less satisfactory from a feature/functionality perspective, but does not generate the performance slowdowns and VM errors.
This bums me out, because I like TextWrangler a lot. And I rather dislike TextEdit. So I'm now looking for a TextWrangler replacement. The obvious candidate is BBEdit Pro, but (a) I don't really want to spend $50 on a text editor and (b) I'm afraid BBEdit Pro might share enough code base with TextWrangler to generate the same VM problem over time. So anyone with thoughts on a good text editor for Mac OS X, please do share them here. Meanwhile, I lump along.