For performance reasons, data on tab pages is not loaded until the tab page is made visible. So you do not have to worry about "heavy" tab controls with lots of tab pages.
There are some exceptions to this rule. For example, if the contents of a tab page contain validation (e.g., required validation on widgets) or conditional formatting, those contents are loaded initially. This is to ensure the correct behavior of the application.
The 'Refresh on show' property just indicates whether the contents of a tab page should be reloaded if you make the tab page visible again. It defaults to true, because that always yields correct behavior. Only set it to false if you know what you're doing.