I've sort of answered your main question here already I think:
http://forums.xmetal.com/index.php/topic,1120I will answer the validation and close questions here to keep them in context.
Validation does take time and will take more time with larger documents. With the test documents that I have seen (sent from either you or a colleague that are about 2MB files) the validation process adds about 2 seconds to the save process. You can measure the amount of time validation takes by pressing F9 to force a validation to be done on its own (without saving).
The close process does not trigger validation directly. If a document has not been saved and you ask to close it then XMetaL asks you to save. If you answer "yes" (save it) then validation is done just prior to the save.