I have been having this problem for a while now- since I don't know when, but only recently I've traced the slowness of my blog to the the_content() function.
I have tried things like disabling the my-hacks.php file (which includes the call to phpGiggle), disabling all plugins which may affect the content, and switching themes. I have also recently installed the Prerender plugin, which has improved the speed of the page overall (I now call the_prerendered_content() in my template), but I do not believe it fixes the problem at its root, wherever it is. Before I installed the plugin, I was getting page render times at around 3-5 seconds on average. Now, the render times are usually around .8-2.5 seconds. I have modified a few php files in the wp-includes directory, but not with anything that would slow them down. I even tried replacing them with the original release files. I have 23 queries on a standard blog page, and the minimal 7 if I create a page only with wp-blog-header.php being included. This blog runs on WordPress 1.5 Strayhorn (stable).
On the same webspace I have a few basic WordPress 1.2 installs (fairly unmodified) that have quick page renders, similar to your typical promised WordPress speed. That is, somewhere around 0.5-1 second.
So, somewhere, the_content() is slowed down.
You can see my blog at http://paxasiana.com -- it's slow, no?