Late notice: we have made quickstart the default; if you need the old behavior, use the new "--autoload" option.

If you start ST/X with a "--quick" command line option, it will not scan the filesystem for autoloadable classes. This results in a MUCH faster startup. However, you must now ensure that the any missing classes are loaded from their package.
Packages can be loaded via the Launcher's "Load Package" menu item, or by evaluating:

Smalltalk loadPackage:<nameOfPackage>

A good place to keep such package-load instructions is "MyWorkspace.wsp", which - if such a file is present in the current directory - is shown automatically in the SystemWorkspace.

Another option is to edit the autoloaded-package's list in the Launcher's "Settings" - "System" - "Autoloaded Packages" list.

The quickstart option is also recommended to check if all required classes are present for a deployed stand-alone binary executable (which can/should not load missing classes dynamically).