Manual:$wgGenerateThumbnailOnParse/de-formal

Details
Allow thumbnail rendering on page view. If this is, a valid thumbnail URL is still outputted, but no file will be created at the target location. This may save some time if you have a  or 404 handler set up which is faster than the regular webserver(s).

Setting this to  is equivalent to setting the   property in   and   to false.