You mention the ditacomponent.dtd so I assume you are authoring DITA content?
Your files are large enough that editing is slow, and you wish to avoid that? That's the main issue?
Files need to be very large (generally) to make editing slow enough to be painful in XMetaL Author Enterprise.
That is even less likely to happen with DITA (since each topic is typically quite small).
You also say you have a custom DTD and customization. Is this a specialized DITA DTD?
If it is a specialized DITA DTD then the Reusable Component feature should work without modification (for authoring and with the DITA Open Toolkit). However, XMetaL Author Enterprise does need to be configured properly to set this up.
For non-DITA documents the Reuse menu and functionality isn't available as it is designed to be used with DITA. So if that is the case I'm not sure how to help.
Or perhaps you want to use files created using ditacomponent.dtd with some other system? In this case (assuming it is a validating XML parser) you should be able to provide it with the ditacomponent.dtd as well.
Or perhaps I'm completely misunderstanding this. I think I need more information or some clarification to help.