Loading times until logon should be less than 2 seconds. Plug-Ins are not yet loaded at this point, so that should not have any effect.
Whithout Plug-Ins and without connecting, PL/SQL Developer should take up 11MB.
There is clearly something wrong. It may be caused by a preference bug in 6.0.2. Can you download version 6.0.3 and let me know if this fixes the problem? It may be necessary to start PL/SQL Developer 6.0.3, change a preference, quit, and restart it.