In order to speed up the performance of the CGI scripts that make up Virtualmin, Webmin can be configured to load some of their Perl libraries into memory at startup time so that they do not have to be read from disk and parsed for every request. However this uses up more RAM - approximately 100M on a typical Linux system.

This setting gives you more control over how much pre-loading is done. The options are:

All libraries
  Libraries for all Webmin modules and plugins used by Virtualmin are loaded. The Webmin server process will consume approximately 100M of RAM.
Only core
  Only those libraries that are part of Virtualmin itself are loaded. The Webmin server process will use around 50M of RAM.
No
  Nothing is pre-loaded - instead, all libraries are loaded on demand. The Webmin server process will use only 15M of RAM.