Every JAR file in the glassfish/modules directory adds to the overhead of server startup. So JAR files that are not OSGi modules should not be in that directory.
This issue is for moving gf-client-module.jar out of glassfish/modules, possibly to the glassfish/lib/appclient directory.
It is unclear how much of a performance gain would result from this. Since this bundle is in the "Installed" state, it isn't actually loaded into the server. So this is a low priority item as far as startup performance time improvement goes.