I tested the speed of four static site generators: Nikola, Pelican, Hexo and Octopress, in a clean environment. Spoiler alert: Nikola won.
Disclaimer: author is a developer and user of Nikola. The test environments used were the same for all four generators.
Nikola v7.6.1, by Roberto Alsina, Chris Warrick and contributors; Python; MIT license
Pelican v3.6.0, by Alexis Metaireau and contributors; Python; GNU AGPL license
Hexo v3.1.1, by Tommy Chen and contributors; Node.js; MIT license
Octopress v2.0, by Brandon Mathis and contributors; Ruby; MIT license (based on Jekyll)