Brief Summary
Closing a window then reopening it causes all tabs to be loaded even if some were suspended. The suspended tab status should be remembered so the window is restored exactly as it was.
Details:
Currently closing a window then reopening it (via History → 'Reopen Last Closed Window' or ⇧⌘T) causes all the tabs to be reloaded which if there are more than a handful creates a noticeable performance impact by trying to reload dozens of tabs at once. If the tabs were previously loaded then that is reasonable and unavoidable, however if they were already suspended then they should remain suspended to prevent the unnecessary (and undesirable) loading.