Codex Generator is search, research and documentation generation tool for WordPress functions.
Plugin draws data from currently defined functions. It cannot see functions from files that are only loaded in specific cases and inactive plugins.
Opcode caching makes things run snappier and discards non-essential information while at it. So PHPDoc (documentation embedded in source) is not available in that case. And this plugin needs it.
Absolutely not! This plugin is mostly meant to be used in development environment. You can run it in production, but you should not compromise performance because of it.