(This bug is specifically for version 1.21wmf5, but that doesn't seem to be an option in the version list, so I selected 1.21-git. I don't know if that's the same or what.)
The site JS loaded from Mediawiki:Common.js is taking a very long time to load/run. The DOM is finished loading about 4-5 seconds before the JS starts running.
Example page: https://en.wiktionary.org/wiki/free