Octopress is quite popular among opensource developers since its include all the features needed to maintain a simple blog plus some extensions like source code syntax hilight, social networks integrations and support to deploy on GitHub Pages and Heroku (both free).
I should say that Octopress isn’t a CMS as many of you can imagine, but a static html pages generator. This may sounds oldie, but Octopress leverage some existing tools, and you will like it.
The following is a Google Webmaster graph, the performance gain was from an average of 600 ms for page retrivial with Drupal (with caches) to an average of 120 ms with Octopress.
To migrate existing posts from Drupal 7, I’ve used a slightly modified version of an existing migration script. Put your mysql connection details and execute it, it will generate a bunch of files to put inside the source/_posts folder of your Octopress.