Summary: Extensions via .config files require reading the .config file...

System.URI

""Using non-standard URI types (e.g. myuri://cool.isit.not) as specified in a .config file of course requires the reading of that config file which can be comparatively costly (but still a fraction of a second). If you need this facility try to initialize the relevant URIs at a point where the delay is less of a problem -- e.g. not at startup time.""