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. The best way to do this is:
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.

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).